dual batteries
Question:
>> >One problem I’ve heard of with tow batteries hard mounted in parallel is > >that at rest, they can possibly self discharge each other. If one is a > >few millivolts lower than the other, some current flows to the lower > >one. The solenoid keeps them apart (normally) when the engine is off, > >so they can’t pull each other down. > Don’t think that can happen….marine configurations are all ‘hard-wired’ > to eachother, with only splits between larger sections of hard-wired > batteries….same with the even more dedicated 2V gel units….and isn’t > all this an external version of what happens internally in a 12v battery > anyway?….each cell delivers 2V, times 6….no internal disbalancing > either.
Oops, the latter is a serial configuration….but the marine style of ‘banks’ of batteries is definately pararallel…. – Hide quoted text — Show quoted text -> And both pure-24v and mixed 12v/24v (system/starting) Land Cruiser diesels > have hard-wired batteries (the 12/24 switching from parallel to serial > while starting of course)….and only pure 24v systems suffer from > potential misbalancing, especially with separate 12v drains without > converter (but there are socalled charge-balancers to tackle that > problem!). >This may very well be true, I’ve heard both versions. I designed my >system for flexibility and reliability, so being able to connect and >separate the two batteries was an important consideration. I tried to >minimize any potential problems in that area by buying two identical >batteries, at the same time, in the hopes of getting some out of the >same mfg. batch. One issue I have with hard-wiring two batteries >together is that if something causes them to discharge, you now have two >dead batteries. If automatically separated, you’ll hopefully only have >one dead one.
Yup, that’s why marine config’s still have separate banks, one for starting, one for recreation (and sometimes a 3rd for a bilge pump, as extra safety in case of a leak), but each bank internally hardwired parallel. — Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!]
Response:
How should you set this up Should you use a relay in-between them, or can you just put them in parallel. I have a GMC Jimmy fullsize if that matters any. Kurt
Response:
– Douglas A. Shrader
> How should you set this up > Should you use a relay in-between them, or can you just put them in > parallel. > I have a GMC Jimmy fullsize if that matters any. > Kurt
On my GMC diesel they simply connect the positive cable from the first battery to the positive post of the second battery and went from the second battery to the starter, no relays. If you are using the original sidemount terminals you just need a longer bolt and a lead spacer to place between the 2 cables, both available from GM.
Response:
> How should you set this up > Should you use a relay in-between them, or can you just put them in > parallel. > I have a GMC Jimmy fullsize if that matters any.
Those are options, a manual switch is another or an isolator. I went with a 200A dual battery solenoid/relay. If you do the relay route, be sure to get one rated for continuous use. Starter solenoids may burn out if left on for a long time. I wrote up my dual battery install on the page, below: — Roger Brown http://reality.sgi.com/rogerb/4×4/CheapTricks/DualBatteries.html "Instructions said "Requires Windows 95 or better, so I installed Linux!"
Response:
My Dodge comes with two batteries in parallel. Be sure to use heavy starter cable between the batteries. This is done to get enough power to reliably start the engine and run the intake heater. You should replace both batteries when one fails with this sort of setup. In Marine and RV applications, various relay in-between configurations get used. These are typically designed so that the charging system can charge both batteries together, but each battery discharges independently (i.e. one for engine starting, the other for everything else). The relays used in the app. do not need power to hold their last setting. It’s even possible to get a dual output alternator and have two completely independent electrical systems. See your local RV or marine (fancier options available for marine) dealer. — 2000 Dodge/Cummins 2500 QC 6spd 4×4 long bed
– Hide quoted text — Show quoted text -> How should you set this up > Should you use a relay in-between them, or can you just put them in > parallel. > I have a GMC Jimmy fullsize if that matters any. > Kurt
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This raises a question(s) for me… What if you want to use your winch and say to get max amps for the winch I want to use both batteries? I have heard everywhere they pull about 400 amps? So say your isolator is good up to as you said 200 amps… Then what? Pray one will pull 200 and the other battery gets the other 200 and hope your unit wont burn out? Can you simply do like others say and use 2 batteries in parallel on EVERYTHING? I was recently working on a boat with 3 batteries. One for starting and engine use and 2 for aux uses. It had one isolator and it separated the 2 from the one. I am not sure… ANYWAYS say I run huge lights on my 4×4 and want to use the winch at the same time.. Should I run all of it off 2 in parallel or run some off one battery and some off the other? I understand why if you separate the circuits you need an isolator but what if 2 batteries are in the exact same circuit and have the same exact loads (or can this never happen except conceptually in my head?)? Scott
– Hide quoted text — Show quoted text -> How should you set this up > Should you use a relay in-between them, or can you just put them in > parallel. > I have a GMC Jimmy fullsize if that matters any. > Those are options, a manual switch is another or an isolator. I went > with a 200A dual battery solenoid/relay. If you do the relay route, be > sure to get one rated for continuous use. Starter solenoids may burn > out if left on for a long time. I wrote up my dual battery install on > the page, below: > — > Roger Brown > http://reality.sgi.com/rogerb/4×4/CheapTricks/DualBatteries.html > "Instructions said "Requires Windows 95 or better, so I installed > Linux!"
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- Hide quoted text — Show quoted text – > This raises a question(s) for me… What if you want to use your winch and > say to get max amps for the winch I want to use both batteries? I have heard > everywhere they pull about 400 amps? So say your isolator is good up to as > you said 200 amps… Then what? Pray one will pull 200 and the other battery > gets the other 200 and hope your unit wont burn out? Can you simply do like > others say and use 2 batteries in parallel on EVERYTHING? I was recently > working on a boat with 3 batteries. One for starting and engine use and 2 > for aux uses. It had one isolator and it separated the 2 from the one. I am > not sure… ANYWAYS say I run huge lights on my 4×4 and want to use the > winch at the same time.. Should I run all of it off 2 in parallel or run > some off one battery and some off the other? I understand why if you > separate the circuits you need an isolator but what if 2 batteries are in > the exact same circuit and have the same exact loads (or can this never > happen except conceptually in my head?)?
Winch is wired direct to my aux. battery with 1/0 welding cable. Main battery and aux. tie in through the solenoid. Assuming similar batteries, I get 200A out of one battery and 200A out of the other, 200+200=400. The solenoid only sees 200A that flows out of the main battery (which is fed by a 160A alternator). Solenoid contacts are rated by the current they will make or break. As long as I don’t try flipping any switches while I’m at full pull on the winch, I should be safe. I have stalled the winch several times (Warn 9000i) so I assume I’ve been at full current, which is indeed 400A, per the spec chart I have: http://reality.sgi.com/rogerb/4×4/4R_bumpers.html#Winch An isolator is probably not the best setup for winching use, they are better for auxiliary power use in a camper or RV for example. The inherent voltage drop in an isolator would serve to limit the current the battery behind it could supply to the load. I’ve heard of dual batt. setups with isolators for normal use and then have solenoids in parallel for high current use. With my setup (Painless Wiring Dual Battery controller) I normally run both batteries in parallel. Solenoid is energized whenever the engine is running in normal mode. I have a 2nd mode when I separate the two batteries regardless of engine power, and a 3rd mode that they are tied together regardless of engine power. One problem I’ve heard of with tow batteries hard mounted in parallel is that at rest, they can possibly self discharge each other. If one is a few millivolts lower than the other, some current flows to the lower one. The solenoid keeps them apart (normally) when the engine is off, so they can’t pull each other down. I also run 12V devices in camp, and this way, I can drain one battery and still have another ready to start the engine. I’ve been running this setup for 2 years now, never failed, and I’ve used all the modes and features numerous times so far.
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>One problem I’ve heard of with tow batteries hard mounted in parallel is >that at rest, they can possibly self discharge each other. If one is a >few millivolts lower than the other, some current flows to the lower >one. The solenoid keeps them apart (normally) when the engine is off, >so they can’t pull each other down.
Don’t think that can happen….marine configurations are all ‘hard-wired’ to eachother, with only splits between larger sections of hard-wired batteries….same with the even more dedicated 2V gel units….and isn’t all this an external version of what happens internally in a 12v battery anyway?….each cell delivers 2V, times 6….no internal disbalancing either. And both pure-24v and mixed 12v/24v (system/starting) Land Cruiser diesels have hard-wired batteries (the 12/24 switching from parallel to serial while starting of course)….and only pure 24v systems suffer from potential misbalancing, especially with separate 12v drains without converter (but there are socalled charge-balancers to tackle that problem!). — Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!]
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Here’s a link to a drawing on how to hook it up. there may be other ways but this looks good… http://personal.nbnet.nb.ca/wquin/DualBatteryDia.jpg — Warren Quin "Most people would die sooner then think – in fact, they do so."
– Hide quoted text — Show quoted text -> How should you set this up > Should you use a relay in-between them, or can you just put them in > parallel. > I have a GMC Jimmy fullsize if that matters any. > Kurt
Response:
- Hide quoted text — Show quoted text ->One problem I’ve heard of with tow batteries hard mounted in parallel is >that at rest, they can possibly self discharge each other. If one is a >few millivolts lower than the other, some current flows to the lower >one. The solenoid keeps them apart (normally) when the engine is off, >so they can’t pull each other down. > Don’t think that can happen….marine configurations are all ‘hard-wired’ > to eachother, with only splits between larger sections of hard-wired > batteries….same with the even more dedicated 2V gel units….and isn’t > all this an external version of what happens internally in a 12v battery > anyway?….each cell delivers 2V, times 6….no internal disbalancing > either. > And both pure-24v and mixed 12v/24v (system/starting) Land Cruiser diesels > have hard-wired batteries (the 12/24 switching from parallel to serial > while starting of course)….and only pure 24v systems suffer from > potential misbalancing, especially with separate 12v drains without > converter (but there are socalled charge-balancers to tackle that > problem!).
This may very well be true, I’ve heard both versions. I designed my system for flexibility and reliability, so being able to connect and separate the two batteries was an important consideration. I tried to minimize any potential problems in that area by buying two identical batteries, at the same time, in the hopes of getting some out of the same mfg. batch. One issue I have with hard-wiring two batteries together is that if something causes them to discharge, you now have two dead batteries. If automatically separated, you’ll hopefully only have one dead one.
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It depends on what you want to use it for… you can directly connect them if you want to use both all of the time (more cranking power, more time listening to your favorite CD w/o the engine, etc.) use a relay-switch on the positive terminal of the second battery to use it as a simple backup…that way it will always be recharged and fresh, but it will only be used when you switch it on (I suggest installing a heavy duty switch on the first battery just to get the truck started using only the second battery, then engage the switch so that the alternator can charge both batteries again).
– Hide quoted text — Show quoted text -> How should you set this up > Should you use a relay in-between them, or can you just put them in > parallel. > I have a GMC Jimmy fullsize if that matters any. > Kurt
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Optima has some info on wiring batteries in parallel: http://www.optimabatteries.com/information/faqs/main.htm#10
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>Hi, >Batery isolator will separate each battery electrically. They won’t kill >each >other. For a real big capacity, have two 6V ones in series. >BTW, 42V electrical system is coming soon.
Btw, don’t get confused by 42V being an odd number compared to 12V….compare it to 14V or 28V charging (12v vs 24v systems), and all is clear….it’s only one third step upwards from a 24v Land Cruiser….;)) — Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!]
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Hi, Batery isolator will separate each battery electrically. They won’t kill each other. For a real big capacity, have two 6V ones in series. BTW, 42V electrical system is coming soon. Tony – Hide quoted text — Show quoted text -> One problem I’ve heard of with tow batteries hard mounted in parallel is > that at rest, they can possibly self discharge each other. If one is a > few millivolts lower than the other, some current flows to the lower > one. The solenoid keeps them apart (normally) when the engine is off, > so they can’t pull each other down. I also run 12V devices in camp, and > this way, I can drain one battery and still have another ready to start > the engine. I’ve been running this setup for 2 years now, never failed, > and I’ve used all the modes and features numerous times so far. > This won’t happen, with one exception: If one of the batteries is bad and > self discharging at a high rate, it will pull the other one down with it if > they are tied together. > It seems to me the NiMH batteries are really the ideal (except $$$) for many > 4×4, rv and marine applications. Compared to lead-acid you get > Much higher max current capacity > Twice as many amp-hours/pound of battery > Withstand several times as many deep cycle charge/discharge events > No hydrogen outgassing > No acid to spill > Works in any orientation (i.e. vehicle upside down) > 33% less charge/discharge losses (80% vs 60%) > Higher recharge current capability > These last two mean faster recovery from high discharges, such as a long > pull on the winch. > But of course, there is a downside to everything: > 5x cost/amp-hour > Not compatible with most existing charging systems. > Can’t get 12.6V. ten cells give 12V, eleven cells give 13.2V, *CAN* get > 25.2V for you 24V’ers > No conveniently packaged complete batteries available > The cost is partly mitigated by the longer life expectancy of the NiMH > battery (brings it down to 2-3X over the life of the battery), but if the > charging system is wrong the NiMH battery will fail quickly. In an RV/boat > the cost is potentially further mitigated by the charging cycle efficiency > because you don’t need as much solar panel capacity to maintain charge. > Lead-acid batteries are damaged by over-discharge and are over-charge > tolerant. > NiMH batteries are tolerant of full-discharge, but are severely damaged by > over-charging (this is the source of the so-called "memory effect"). > The 0.6V error probably only matters to the charging system. IIRC the > charging voltage for 12.6V lead-acid is higher than you would get for a > 13.2V NiMH battery. The extra charging voltage required by lead-acid > batteries is where much of the 33% charge/discharge cycle efficiency > difference comes from. > Soldering a pile of 1.2V "F" size cells (roughly 2 "D" cells end-to-end) > together seems like a major effort and potentially hazardous work. > Has anybody ever tried to wire these up? > —
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>x-no-archive: yes >Note: Respond to news group as address is mugged to prevent spam >| This won’t happen, with one exception: If one of the batteries is bad and >| self discharging at a high rate, it will pull the other one down with it if >| they are tied together. >*There is no way to exactly match the batteries. One will always be the weaker >’twin’ and while you can get away with paralleling them there is a cost in >battery life.
But much less than in a 24V serial setup….that is why Toyota went to twin-serial (12V) in later series diesel Land Cruiser, with 24V starting-only, instead of twin-parallel (true 24V)….much less battery problems (and I am actually very biased towards 24V, because of electrical(winch) performance!….luckely there are charge-balancers to solve the typical 24V unbalance (as well as 12v non-converted/single-battery drains)). >* As to NiCd. The charging system for lead-acid batteries is not compatible >with that required for NiCd. (Lead-acid is voltage limited whereas NiCds are >current limited) >* Second and perhaps most important for anyone living out side of South Florida >is NiCd have p***-poor performance when cold and vertually no performance below >freezing.
I am pretty sure their performance is close to lead-acid, if not better….going from +20C to -20C with lead-acid means a capacity drop of 50% (multiply with a 2x more sluggish engine and you are down to 25% of original starting performance)….for NiCd spec’s see camera-related message below: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Priority: normal > Hi All, > For everyone who uses PB-E1 on their EOS1/1N/3 with NiCad E1 batteries : > Could you tell me if these batteries accumulate a "memory " effect – so it > hold less and less charge and if so – how to deal with it? Or is there any way > to deal with it besides buying a new one?
I recommend you first read a few chapters on my homepage about this so called ‘memory’ effect: http://www.a1.nl/phomepag/markerink/mainpage.htm It’s quite a bit different than most manufacturers want you to believe….nothing less than a very persisting urban myth, all based on some very controlled & repetitive NASA experiments. Btw, did anyone notice the interesting EOS-1V table of battery-type vs number of films, at +20C and -20C? It clearly shows that NiCd still has an advantage over NiMH at such extreme temperatures, despite being less than half the nominal capacity in mAh…. (Number of 36-rolls until complete empty/shut-off, last 2 collumns added by WJ) Remaining Nominal capacity +20C -20C capacity (estimate/market-average) 2CR5 50 12 24% 1300mAh 8x Alkaline 85 5 6% 2000mAh 8x Lithium 120 50 42% 3000mAh 8x NiCd 35 24 69% 500/600mAh 8x NiMH 70 20 29% 1000/1200mAh I knew NiCd was better than NiMH in cold in a relative context, but this even proves it is better in an *absolute* context too…. Also note that the above Canon table is based on NiCd’s from 5-600mAh, while currently cells exist that are twice this capacity (1100-1200mAh). This puts them on the same level as Lithium under those arctic conditions. xxxxxxxxxxxxxxxxxxxx — Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!]
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– Hide quoted text — Show quoted text -> One problem I’ve heard of with tow batteries hard mounted in parallel is > that at rest, they can possibly self discharge each other. If one is a > few millivolts lower than the other, some current flows to the lower > one. The solenoid keeps them apart (normally) when the engine is off, > so they can’t pull each other down. I also run 12V devices in camp, and > this way, I can drain one battery and still have another ready to start > the engine. I’ve been running this setup for 2 years now, never failed, > and I’ve used all the modes and features numerous times so far. > This won’t happen, with one exception: If one of the batteries is bad and > self discharging at a high rate, it will pull the other one down with it if > they are tied together. > It seems to me the NiMH batteries are really the ideal (except $$$) for many > 4×4, rv and marine applications. Compared to lead-acid you get > Much higher max current capacity > Twice as many amp-hours/pound of battery > Withstand several times as many deep cycle charge/discharge events > No hydrogen outgassing > No acid to spill > Works in any orientation (i.e. vehicle upside down) > 33% less charge/discharge losses (80% vs 60%) > Higher recharge current capability > These last two mean faster recovery from high discharges, such as a long > pull on the winch. > But of course, there is a downside to everything: > 5x cost/amp-hour > Not compatible with most existing charging systems. > Can’t get 12.6V. ten cells give 12V, eleven cells give 13.2V, *CAN* get > 25.2V for you 24V’ers
Thats no biggie as most 12v loads are designed with a wide tolerance since the voltage will be around 12 when being used and 13 when the vahicle is charging > No conveniently packaged complete batteries available
Buy them from electronic catalogs (mouser, digikey, etc) you can get solder tabbed batteries making it easy to wire them up or you can get them already built and packaged in any voltage / shape / size you want. I used to repair laptop batteries this way but haven’t yet found a good source of L-Ion batteries for modern laptops. > The cost is partly mitigated by the longer life expectancy of the NiMH > battery (brings it down to 2-3X over the life of the battery), but if the > charging system is wrong the NiMH battery will fail quickly. In an RV/boat > the cost is potentially further mitigated by the charging cycle efficiency > because you don’t need as much solar panel capacity to maintain charge. > Lead-acid batteries are damaged by over-discharge and are over-charge > tolerant. > NiMH batteries are tolerant of full-discharge, but are severely damaged by > over-charging (this is the source of the so-called "memory effect").
I have always heard its best not to over discharge NiMH’s either, most faq’s say to use alkaline in things like flashlights that have the potential to over-discharge > The 0.6V error probably only matters to the charging system. IIRC the > charging voltage for 12.6V lead-acid is higher than you would get for a > 13.2V NiMH battery. The extra charging voltage required by lead-acid > batteries is where much of the 33% charge/discharge cycle efficiency > difference comes from. > Soldering a pile of 1.2V "F" size cells (roughly 2 "D" cells end-to-end) > together seems like a major effort and potentially hazardous work.
see above, find them with solder tabs or pre made > Has anybody ever tried to wire these up? > —
Heres another thought. Get a small truck (i.e. S-10,Ranger) put one of the metal boxes in the bed and fill it with batteries. Add in a second alt just for the other batteries, then connect a forklift charger type plug onto the box and the other plug to your garage. Commute about an hour toand fom work each day and pak in your garage and plug up when you home. Then wire the house for 12v. This way you charge the batteries at a very slight decrease in MPG but you charge the batteries enough to run the house overnight and charge them up on the trip to/from work. Eugene – Hide quoted text — Show quoted text –
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– Hide quoted text — Show quoted text -> One problem I’ve heard of with tow batteries hard mounted in parallel is > that at rest, they can possibly self discharge each other. If one is a > few millivolts lower than the other, some current flows to the lower > one. The solenoid keeps them apart (normally) when the engine is off, > so they can’t pull each other down. I also run 12V devices in camp, and > this way, I can drain one battery and still have another ready to start > the engine. I’ve been running this setup for 2 years now, never failed, > and I’ve used all the modes and features numerous times so far. >This won’t happen, with one exception: If one of the batteries is bad and >self discharging at a high rate, it will pull the other one down with it if >they are tied together. >It seems to me the NiMH batteries are really the ideal (except $$$) for many >4×4, rv and marine applications. Compared to lead-acid you get > Much higher max current capacity > Twice as many amp-hours/pound of battery > Withstand several times as many deep cycle charge/discharge events > No hydrogen outgassing > No acid to spill > Works in any orientation (i.e. vehicle upside down) > 33% less charge/discharge losses (80% vs 60%) > Higher recharge current capability >These last two mean faster recovery from high discharges, such as a long >pull on the winch.
The last and first one also apply to Optima batteries…. NiCd does even better on both. >But of course, there is a downside to everything: > 5x cost/amp-hour > Not compatible with most existing charging systems.
- very vulnerable to overcharging….much more than NiCd (*the* main reason why manufacturers can claim lack of memory effect….the high-end chargers necessary to prevent overcharging also prevent most symptoms of ‘memory effect’ (which is an urban myth in itself, for both types; see my homepage for a long list of Battery-FAQ’s debunking this myth (and a few more): http://www.a1.nl/phomepag/markerink/mainpage.htm (a search on ‘battery’ will get you there the fastest….;)) > Can’t get 12.6V. ten cells give 12V, eleven cells give 13.2V, *CAN* get >25.2V for you 24V’ers > No conveniently packaged complete batteries available
- high rate of self-discharge, much higher than NiCd…. >The cost is partly mitigated by the longer life expectancy of the NiMH >battery (brings it down to 2-3X over the life of the battery), but if the >charging system is wrong the NiMH battery will fail quickly. In an RV/boat >the cost is potentially further mitigated by the charging cycle efficiency >because you don’t need as much solar panel capacity to maintain charge. >Lead-acid batteries are damaged by over-discharge and are over-charge >tolerant.
Normal lead-acid is….gel-cell and optima’s are much less. >NiMH batteries are tolerant of full-discharge, but are severely damaged by >over-charging (this is the source of the so-called "memory effect").
See above….;)) >The 0.6V error probably only matters to the charging system. IIRC the >charging voltage for 12.6V lead-acid is higher than you would get for a >13.2V NiMH battery. The extra charging voltage required by lead-acid >batteries is where much of the 33% charge/discharge cycle efficiency >difference comes from. >Soldering a pile of 1.2V "F" size cells (roughly 2 "D" cells end-to-end) >together seems like a major effort and potentially hazardous work. >Has anybody ever tried to wire these up?
I don’t even think you *can* charge NiCd nor NiMH under massively varying loads….there’s no way a charge-computer can detect voltage drops securely….even the slightest irregular drain while charging will make the system go into tilt-mode….8-)) The beauty of lead-acid/gel is that it only takes a continuous voltage (of 13.8V)….so you only need a voltage regulator that maintains this 13.8V, regardless of drains…. — Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!]
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>Here’s a link to a drawing on how to hook it up. there may be other ways but >this looks good…
Here is another alternative. This company has an excellent isolator, combiner. http://www.hellroaring.com/ Mike Nellis
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> One problem I’ve heard of with tow batteries hard mounted in parallel is > that at rest, they can possibly self discharge each other. If one is a > few millivolts lower than the other, some current flows to the lower > one. The solenoid keeps them apart (normally) when the engine is off, > so they can’t pull each other down. I also run 12V devices in camp, and > this way, I can drain one battery and still have another ready to start > the engine. I’ve been running this setup for 2 years now, never failed, > and I’ve used all the modes and features numerous times so far.
This won’t happen, with one exception: If one of the batteries is bad and self discharging at a high rate, it will pull the other one down with it if they are tied together. It seems to me the NiMH batteries are really the ideal (except $$$) for many 4×4, rv and marine applications. Compared to lead-acid you get Much higher max current capacity Twice as many amp-hours/pound of battery Withstand several times as many deep cycle charge/discharge events No hydrogen outgassing No acid to spill Works in any orientation (i.e. vehicle upside down) 33% less charge/discharge losses (80% vs 60%) Higher recharge current capability These last two mean faster recovery from high discharges, such as a long pull on the winch. But of course, there is a downside to everything: 5x cost/amp-hour Not compatible with most existing charging systems. Can’t get 12.6V. ten cells give 12V, eleven cells give 13.2V, *CAN* get 25.2V for you 24V’ers No conveniently packaged complete batteries available The cost is partly mitigated by the longer life expectancy of the NiMH battery (brings it down to 2-3X over the life of the battery), but if the charging system is wrong the NiMH battery will fail quickly. In an RV/boat the cost is potentially further mitigated by the charging cycle efficiency because you don’t need as much solar panel capacity to maintain charge. Lead-acid batteries are damaged by over-discharge and are over-charge tolerant. NiMH batteries are tolerant of full-discharge, but are severely damaged by over-charging (this is the source of the so-called "memory effect"). The 0.6V error probably only matters to the charging system. IIRC the charging voltage for 12.6V lead-acid is higher than you would get for a 13.2V NiMH battery. The extra charging voltage required by lead-acid batteries is where much of the 33% charge/discharge cycle efficiency difference comes from. Soldering a pile of 1.2V "F" size cells (roughly 2 "D" cells end-to-end) together seems like a major effort and potentially hazardous work. Has anybody ever tried to wire these up? —
