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eromlignod (Mechanical) (OP)
26 Sep 06 17:36
Hi Guys:

If I want to charge, say, ten 12V lead-acid batteries all connected in parallel, can I use a single battery charger?  Would this just mean that it will take ten times as long for the batteries to charge as for a single battery?  Will there be problems with the charger eventually overcharging the batteries?  Other problems?

Thanks for your replies.

Comcokid (Electrical)
26 Sep 06 18:26
I would be concerned about connecting batteries in parallel for charging. The weakest battery is going to get charged by all the other batteries plus the charger, and if that weak battery actually had a few bad cells reducing it's voltage, you could end up with a lot of current running through it. Think boiling acid and possible battery explosion.

Now, if it was 10 batteries of similar age, same amp/hour rating, similar discharge, you could charge them in parallel. But as soon as you had them charged, you would want to immediately disconnect them from each other. Charging ten batteries in parallel would take 10x as long to charge. Overcharging them would be less an issue.

Battery isolators are made for when batteries must be hooked in parallel. These consist of diodes or a circuit to keep current going only one direction. Otherwise, batteries are hooked in series to charge, and a higher voltage charger is used.

Situations where lead-acid batteries are hooked in parallel, at least the situations I am aware of, are all situations in which is done only momentarirly. Examples - when jumping a car battery, or on a RV where there is a momentary push-to-hold switch controlling a relay that lets you hook the engine and house battery together when one is dead. In both cases, the cables get warm from the current flow from one battery into the other, but not a problem as it is only for a moment.
felixc (Electrical)
26 Sep 06 19:26
You can also ask the question to the Battery Engineering Forum.  I've asked a similar question in the past, and apparently paralleled lead-acid batteries are quite common.  On trucks notably.  Batteries have to be of the same brand and lot to reduce weak links.  But as the charge is in direct proportion with the voltage, after an initial current exchange until all the voltages balance, the capacities will just add up without conflict.  The one with less capacity will not overcharge, as its voltage will just follow the others, same for discharge.  So in theory yes if you have a good battery charger topology you could charge your batteries in parallel and see them as one big battery.  Like parallel capacitors.
Practically (and eventually legally) a problem with parallel batteries will happen if a cell gets shorted in one of the batteries.  Then the voltage/charge curve will not be valid for that one battery and you can run into nasty and dangerous problems.  (Boom!)  In addition to a good charger system you will need good maintenance procedures.  This type of problem will be avoided with a battery isolator, as Comcokid mentions.
The people in the other forum might be able to give you names of available and proven (maybe even UL approved?) systems for parallel batteries.
Helpful Member!  itsmoked (Electrical)
26 Sep 06 19:59
Here is my 2bits;

If those batteries are all in a fixed system fine. Do it.  If you are talking about random batteries dragged in and added to a charger circuit. NO NO NO NO!! You are risking your life.  Adding a bunch of random batteries means you will someday add a bad battery that will suddenly be the recipient of thousands of flash amps of charge from its mates.  The battery will pop like a water balloon. (Read permanent disfigurement and an blindness)

Also charging is a bit of a science.  The charger will end up mis-charging the stack because of confusion by the currents and voltages the charger sees during swap outs etc.

Excellent 3 stage chargers are cheap now.  They are switcher based and are the size of a wall wart.  You can get one for each battery.

Keith Cress
Flamin Systems, Inc.-

eromlignod (Mechanical) (OP)
26 Sep 06 22:02
They will all be the same type and age (new).  The same batteries will always be in shunt with each other. As long as they all discharge and charge together as a unit I guess it's sort of like one big battery.

Apparently I will need some high-current diodes between batteries to keep the current in the right direction.

I have seen electric forklifts that run on multiple batteries (maybe eight or so).  Are those in parallel or series?

VE1BLL (Military)
26 Sep 06 22:25
Large UPS systems have lots and lots and lots of lead acid batteries wired in permanent parallel.

I'm trying to figure out what the 'high-current diodes' would be doing considering that each battery should have power going in both directions (one direction at a time) either for charging and discharging. If you had diodes on the charging side you would probably mess-up the careful voltage calibration of the charger.

eromlignod (Mechanical) (OP)
26 Sep 06 23:19
The diodes weren't my idea, they were Comcokid's.  Are you saying they are unnecessary in my case?

felixc (Electrical)
26 Sep 06 23:24
Will your application require large peak currents, like starting a motor.  I'm not even sure that there exists a diode to handle such currents.  Furthermore the voltage loss at the diodes, at such currents, may become a problem too.
eromlignod (Mechanical) (OP)
27 Sep 06 1:04
It will be a high current application, but I've seen rectifiers that are good to thousands of amps, which is much more than I would need.

itsmoked (Electrical)
27 Sep 06 3:30
You do not need any diodes.  You are correct, if they are all the same, it is just like some mondo battery.

You should always consider fuse protection in non starting battery systems.

Forklifts often run a bunch of batteries in series to get the voltage up for smaller wire and motors.  They often use multiple single cell batteries. They would use 11 or 22 cells for 24V or 48V systems.

Keith Cress
Flamin Systems, Inc.-

OperaHouse (Electrical)
27 Sep 06 3:39
The only problem I see is that periodically batteries need an equalize charge.  In this a battery is chargeged to a higher than normal voltage to create bubbles that circulate the fluid.  If it is a small charger, ten parallel batteries may soak up enough current and never get to that state.  It has been mentioned that this bank of batteries can produce a lot of current and be dangerous.  These batteries should be fused seperately.  This would also give you a method to charge each battery seperately on occasion.
bogeyman (Aerospace)
27 Sep 06 3:48
If you are going to discharge the entire battery system at heavy current ensure that the external resistance of the connecting cables is equal to ensure that each battery of your multiple batteries is discharged equaly.
Try to use equal legnths of the same wire type to a star point via an appropriate large fuse link.
Helpful Member!  jimkirk (Electrical)
27 Sep 06 7:56
It takes about 12 to 16 hours to charge a lead acid battery, charging 10 in parallel shouldn't take a week.  Given all the appropriate concerns about paralleling batteries people have mentioned; same type, same age, et cetera, in general you can just consider it a battery with 10 times the capacity.  If you then get a charger appropriately rated for ten times the charge current, you can theoretically charge them at the same speed as the single battery.  

I'd ease off a bit as a safety margin, and you might consider some sort of current sensing in series with each battery (make sure they can handle your load currents) that would sense any discrepancy in the current and shut off charging and notify you of a problem.

Allegro MicroSystems makes some neat Hall effect based current sensors; isolated output and range up to +/-200 amps that might be appropriate.

See for more info on charging.
felixc (Electrical)
27 Sep 06 8:50
Great web site jimkirk!
eromlignod (Mechanical) (OP)
27 Sep 06 9:44
Thanks for all the great posts so far!

So should I use one big fuse at the main output of the battery bank, or is it also necessary to fuse each battery individually (between its terminal and the main bus)?

itsmoked (Electrical)
27 Sep 06 14:51
Each with it's own.

A common battery failure mode is for a plate to mechanically fail and fall shorting a cell in a battery.  This then means you suddenly have a 10V battery in parallel with 9 other 12V batteries.  They promptly try to charge the this new 10V battery up to 12V.... They can't.  Either they all are promptly discharged to 10V or the failed battery goes into to a brief low earth orbit.  Individual fuses will prevent this.  Two batteries might result in just two dead batteries but with your number of batteries that isn't likely.

Keith Cress
Flamin Systems, Inc.-

itsmoked (Electrical)
27 Sep 06 17:04
Hey!  What are these all running??  This in a cruising boat?  For starting? For deep discharge? What? What?

Keith Cress
Flamin Systems, Inc.-

eromlignod (Mechanical) (OP)
27 Sep 06 17:26
A pure resistive load with no inductance.

ScottyUK (Electrical)
28 Sep 06 13:17

If you are using cable to interconnect the batteries, consider taking a connection from each end of the positive paralleling  bus and from each end of the negative bus. This will reduce the effect of volt-drop in the bus, unless you're using something really heavy for the paralleling links like a main earthing bar for a building or an old section of busbar from an industrial switchboard. Taking the connections from opposite ends so if (say) the positive connection to the battery bank is adjacent to battery #1, make the negative connection adjacent to #10 to equalise the conductor path lengths. Make sure your fuselinks are of the same brand and rating, and consider using bolt-in links without a carrier to reduce volt-drop and minimise contact resistance.

  Sometimes I only open my mouth to swap feet...

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