Multiple lead-acid charging

Hi guys:
I'm an ME and have run across an application where I need to charge multiple lead-acid batteries connected in parallel. The batteries
will discharge at near their maximum continuous current rate when in use, but will be used relatively infrequently. In other words, after a discharge, they may have many hours, or even days, to recharge before they're used again, so I don't need an aggressive charge rate. They will always be charged and discharged together, as if they were one large high-capacity, high-current battery.
What is the best way to charge multiple batteries (say 10 or so)? It would be preferable to me to use off-the-shelf items and I would like to use ordinary 120V power. Could I use several individual chargers in shunt? Do they already make chargers for such an application? Can the batteries be used while they are charging, if necessary?
Thanks for any help you can give me.
Don
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Need a few more details such as the voltage and amp hours to start also the load current, type of battery and cadence before anyone can make suggestions. Dan
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eromlignod wrote:

AFAIK, it's generally not easy on batteries to operate them in series-parallel like this, particularly in charging. I believe (but wouldn't swear) that the problem is worse for NiCd and NiMH batteries, but in essence you'll end up with 'good' batteries 'stealing' all the charge current. At best the weak batteries will never get fully charged and will end up damaged in discharge; at worst the 'good' batteries will get overcharged, and turn into weak ones.
Ideally you would split each battery out into individual cells, parallel each rank of cells into a big 2V 'cell', then connect all the ranks in series. Clearly this isn't something you can do with off the shelf sealed units, but keep in mind that this is what you're failing to do.
I assume you just can't find a really big battery?
Ideally you'd charge each battery individually and discharge them together. This would require some fancy electronics to manage the process, but the job is more complex than it is esoteric. If you did go the fancy electronics route, switching over to the batteries in the middle of a charge would be fairly easy; actually _using_ the batteries at the same time they are charging would require even fancier electronics.
Several individual chargers in shunt would act like one charger, and may well act like one little charger instead of one big one. Of your many choices, this one is probably just a good way of wasting money.
The book "Rechargeable Batteries Applications Handbook", written by the technical marketing staff at Gates Energy and published by Butterworth-Heinemann, has never steered me wrong.
--

Tim Wescott
Wescott Design Services
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Is there I way I could arrange rectifiers to contol the leak between batteries?
I'm not very good at doing ASCII schematics, but I'll try to explain. If all of the positive posts of the batteries had forward diodes to a common bus, forcing the current to only travel in the discharge direction, would that prevent charge transfer between batteries?
Obviously that wouldn't allow re-charging, but couldn't I also have reverse diodes that only allow inward (charging) current flow that connect to a separate "charging" bus?
...or are these just wacky fantasies.
Don
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you could or thyristors to control all whats the point its not just the ballancing of the paralell string its simple construction of cells they will accept and discharge differently only slightly but will get worse each time.

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eromlignod wrote:

Afraid so. Losing battery voltage (and watts) in series diodes isn't normally a good idea.
It would be possible to produce some complex design that isolates individual batteries and charges them, whilst leaving the remainder of the array connected to the load via, say, power FETs. However, if the batteries are closely matched, none of this is necessary - just routine maintenance will do the job.
-- Sue
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While we're on the subject...
What determines the maximum current that a lead-acid battery can supply? They are rated in amp-hours and cold-crank-amps; But for how long can they withstand a large current?
If, for example, I have an ordinary 100 A-h, 600 cca car battery, what's the most current it can continuously provide?
Don
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eromlignod wrote:

The manufacturer's data sheet tells all. Here (with any luck) is that for a Trojan 125:
http://www.trojanbattery.com/Products/T-125Plus6V.aspx
Note that it will provide 25A for almost 500 minutes, but 75A for 132 minutes and not the 162 that could be expected.
The most current a battery can provide is the short circuit current. But it won't produce that for very long at all...
-- Sue
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The internal resistance of the cell and terminal voltage determins max amp supply.amp hours rating is a current / time .i.e. 100ahrs @5hr rate or 10hr rate ....car batteries are designed with very low internal resistance i.e thin plates packed tight to give high ist currents......motive power higher resistance thicker plates more active material low inst currents but supply currents for much longer .....your 100 ahr battery would be something like 80% of 100ahr @5hr rate 80/5 amps for 5hrs but it will supply 600amps for a few minutes.
pat

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pat wrote:

Starting batteries have porous plates to increase surface area which increases the amperage capacity. Deep cycle batteries use smooth plates which are less prone to sulfation but the capacity is lower. The number of plates is fixed in either case because the voltage depends on the number. It's the surface area that determines capacity.
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rate 80/5

all batteries have porous plates, increased surface area is accomplished in car batteries by have many thin plates.deep cycle batteries have predominatley moved to tubular grids producing an increase in surface area and much thicker plates...more active material....thus the capacity is not lower ....voltage is the same if you have 2 neg and 1 pos plate or 15neg and 14 pos plates....the only way you alter voltage without getting into temp....s.g ....is by series the cell up.capacity is determined by active suface area and the rate of coversion of active material into pbso4 thats why the s.g goes down as the cell becomes dischrged.
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forgot to mention temperature most batteries ar rated at a specific temp norm 30 C below less a/h above more but only to a point.....

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Poblem recharging parallel strings of batteries is that each string will discharge and recharge to slightly different levels, when i've done this in the past it pays to limit the discharge to 80% a single charger of the right output is able to do the main recharge but at regular intervalls they will need an equalising charge....i.e. manual finnishing with someone checking the s.g of the cells,temp and pref top of charge volts. otherwise each time some active material will remain as pbso4 lead sulphate and will be unrecoverable and over time this will result in lost capacity. if you got specs ect could give more detail. good luck.

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eromlignod wrote:

The biggest set-up I have been responsible for had arrays of 6 Trojan T125s in parallel, series connected to give 24v.
No problem - other than getting scared stiff whenever I went near that lot ;)
I always ensured that all the batteries that were to be connected in parallel were brand new, unused and the same make, model and batch number and were individually fully charged before connecting up. I never added a battery in parallel with an existing array. I never replaced a single battery but always the complete array it was a member of. All cells were checked weekly and equalisation used as necessary.
The charger must be one designed for such a system - eg it must have have an equalisation setting.
Yes they do make chargers especially for such applications.
Yes, the battery manufacturer (and the charger manufacturer, Victron, IIRC) were very helpful. Including helping design the wiring harnesses and circuit protection.
That one ran off 240v power. I expect that there are 120v equivalents - try a search on "inverter charger".
It isn't something to take on without expert advice - there is one heck of a lot of power waiting to blast gallons of boiling acid all over you, if you get it wrong.. The battery room itself needs proper design in regards to ventilation and acid containment, if something does go wrong. The batteries do need regular monitoring and easy access.
-- Sue
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slight exageration over the boiling acid scenario.....but certainly no sparks flames or anything like....or hydrogen bang....

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pat wrote:

It may easily be a "misremembering" - of something that happened a very long,long time ago. The story at the time was that the plates in a cell in such an array warped and touched each other. The result of which, in an array of batteries connected in parallel, I'll leave for you to consider. It isn't something that I would want to be next to.
Battery cases are built of sterner stuff, these days..
-- Sue
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the more likley chance is the sepeators breking down and this does occur with frequency but in large cells and the amount of liquid it takes a lot worse case senario i have seen is boiling off electolyte and melt down of the lead components.this happend due to charger runaway.

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Yeah, when I was studying in Kozani, we visited the North Load Distribution Centre (utility) and in the battery room of the UPS, they had special switches to avoid dangerous arcs, and even a specially designed bidet (!) so that you could wash your eyes if they were contaminated with (sulphuric) acid.
--
Tzortzakakis Dimitrios
major in electrical engineering
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Palindrome wrote:

You'd be surprised what you can get used to.
Try climbing on top of 128 cell battery rated for 5250 Ah at the 3-hour rate. Navy submarine batteries are huge. ;-)
daestrom (former Chief Electrician's Mate, Submarines)
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Aha! So, conventional submarines were the first hybrid vessels?
--
Tzortzakakis Dimitrios
major in electrical engineering
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