Combining battery question.

Yes.... you are correct.... PARALLEL

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Yes, with a note of caution.... The batteries should both be in the same condition, i.e., brand new, or having been used in the same type of service for the same amount of time. The reason for this is to avoid the possibility of one battery discharging into the other, and to make sure that they both supply current into the load equally.. Suppose that one battery is new, and the other has seen many discharge/charge cycles in its life. That means that the used battery may have deterioriated somewhat so that its internal resistance is higher than the new battery. When the batteries are connected in parallel, the new battery will have to supply a greater amount of current to the load, possibly damaging it. It also may not accept a charge as well as the new battery, and may cause the new battery to be overcharged.

Batteries aren't usually rated in amps. Do you mean they are rated at 7 ampere hours (ah)? In that case, using them in parallel will give you 14 ah, more or less. The fact that the batteries can never be completely identical may introduce some oddities.

Or perhaps you mean that you have a device that draws 7 amps that you are running off one battery and you want to know if you can use two batteries in parallel to run two of the devices? In that case I would ask Why not run each device off its own battery?

If lead-acid you'll probably get away with it - although it won't be quite as good as a single battery of the correct capacity. But it won't work well with Ni-Cads, etc.

I do not agree with the reply you were provided by DaveM, although I do agree with his note of caution if using the batteries in parallel.

Although it is not clear exactly what you mean by "combining" the batteries (this can be either in Series or Parallel), if the two 12V batteries are placed in series, then they will produce 24V nominal voltage with no increase in the maximum current they can supply. If the batteries are combined in parallel, then the voltage will still be 12V, but the maximum current they can supply will double.

That said, the ampere rating you provide is most likely not a true maximum current (in amps as you noted), but is probably a CAPACITY rating for the battery and if so, would be supplied in amp-hours. For the 12V battery you cited, a 7AH rating would mean that hypothetically you could supply a current of 7 amperes for a period of 1 hour. Likewise, if you were supplying, say 700 milli-amps, then you would expect to be able to do so for about 10 hours. (This is assuming that the internal resistance of the battery and its construction/specifications would support these currents for this period of time - it may be that the maximum spec for current is substantially less than the 7A I used as an example.) The bottom line here is that when the batteries are in series, their combined AH rating will not change. When used in parallel, the AH rating will double. If you do the math you will see that the energy supplied (in watt-hours, where watt = volts * amps) is the same as the theory of energy conservation would dictate must occur.

Bob

service

To: DaveM cc: James Hahn Bob Shuman

The simplified answer to DaveM is: YES .... your hunch was correct, hook up the batteries in parallel which, in a perfect world, will give you the desired rating of 12v 14A.

electricitym . . .

Dave Plowman: Judging from the original rating of 12v 7A..... I would say with confidence that this is NOT a NiCad or NiMH.... but more than likely a gel-cell (aka: Lead Acid) therefore hooking them up in parallel is quite acceptable. electricitym

Tho', I would not re-charge them in parallel. Recharge'em 1 at a time.

Jonesy

That's a fair guess if it had been stated as 7Ah. However, it was just a side comment. ;-)

Allodoxaphobia: The reply posting by electricitym is CORRECT. Absolutely NOT a problem to charge lead acid (gel-cell) batteries in parallel .... ...this is done ALL THE TIME with automotive and RV/Trailer batteries. When the towing vehicle is running it is charging both the engine battery AND the trailer battery at the same time..... they are in fact connected in parallel via a mechanical relay/solenoid or a solid state isolator.

-- Best Regards, Daniel Sofie Electronics Supply & Repair

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Gel cell and lead acid ain't the same - and you don't get gel cell in cars as a rule.

SLA - sealed lead acid - which use a gel as the electrolyte - should be charged with a maximum voltage of 13.8. They can't gas in the same way as a liquid electrolyte type can, so mustn't be overcharged.

Dave Plowman: ........ Many motorcycle shops will substitute sealed Gel Cell batteries in dirt bikes to avoid spilling acid when the bike is laid down...... and motorcycle charging systems operate much the same as automotive charging systems..... running anywhere between 13.2 to 14.7volts depending on the load and the engine RPM.

-- Best Regards, Daniel Sofie Electronics Supply & Repair

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Yes - as will AWD vehicles used for off road where they may overturn. But using a charging system designed for 'wet' lead acid where the voltage may go up to well over 14 volts will result in a very short life.

However, if a bike or AWD is regularly being overturned, the cost of a new battery is likely to be insignificant.

Agreed. Charging gel-cells at over rated voltage kills the capacity. The battery will still have proper voltage, but if you do a capacity test, it will be drastically less than rated. We see this all the time on ATC-601's. Aircraft mechanics are told to keep equipment on chargers at all times, which is fine for old NiCd packs, but these charge at 14 volts and constanly come in with ruined packs. They won't last more than 5 minutes at rated capacity. I've been told that the cause is because they can't "vent", as you say. They seem to work perfectly as a replacement for lead-acid, as in they charge in exactly the same manner, but they do lose their capacity if charged at too high a voltage too often. As Daniel Sofie says, they put them in dirt bikes, which makes sense. They charging system charges the batteries just as it should, but I can guarentee that it won't last as long as a lead acid would in its place if it wasn't vibrated and jarred to death. The capacity loss is just an acceptable sacrifice for no battery acid spilling. Possibly a bold statement, just my thoughts. I bet the gel-cells still last at least a year in that situation, but they probably have to be replaced prematurely due to capacity loss. But, there is no perfect battery pack, and gel-cells are definantely nice. Steve

Yes. The accepted way is to charge at constant voltage of 13.8. This, of course, takes more time than blasting a non sealed liquid electrolyte type

Or anywhere where they might be overturned. So called sealed no maintenance liquid ones still have a vent and can leak if overturned.

The big snag for auto use is that they can't be re-charged so quickly without shortening their life. But for many other applications they're still quite competitive.

I'm a sound engineer and will use them where I've got a reasonably static location where I can't easily get mains. One car sized one will run all my gear all day with capacity to spare. And re-charge overnight. Ni-Cads, etc, are of course used where weight and space are a consideration, but the overall running costs of the much higher initial price per Ah - and the lack of large capacity ones at a sensible price, so more changes during the day - make the ancient technology of lead acid still attractive.

Gel cellls are lead acid. What makes them burn out so easily when overcharged is that they use "micro-cell" technology instead of the older lead plate arrays. Micro-celling give them greater density but requires a bit of care when charging. My SLA smart charger starts out at a peak voltage rating of 13.1 with my larger SLA's and 12.3 with my smaller packs. As for life in four wheelers/dirt bikes you usually get 2-3 years of use if properly maintained.( trickle charged during the winter months or periods of non-use greater than 30 days.) They do make great packs for use in all sorts of apps. I use them with my video cameras and lights as well as with cordless power tools.( I have a 12v 8ah pack on a belt pack that I use for all my 12v power tools, drills, skil saws etc... and they'll run most all day on on charge. Much cheaper than the oem nicad packs.