Battery Bank Charging Query

If I were saying garbage like that, I too would want to post anonymously!

I don't have enough experience in this area to give a good answer to the question, but I do have enough experience to know that post #1 is NOT a good answer!

This is very little info to start with. To charge batteries you need a good controller with MPPT. This will take care of the optimal charge in different situations. If you want to keep it simple, you install your panels and connect groups in series so that you have more than 48 volts available to charge the batteries. My 48 volts groups are theoretically 4 X 12 volts, but I have more than 60 volts on a sunny day. This of course kills the batteries unless you protect these against overcharging. On the other hand it means also that there is more amperage available to connect eventually a bigger bank of batteries. To get more specifics, check the supply data of your panels. (current and voltage supplied)

Supply more information:

What is the composition of the banks: type of batteries - differences in cell capacities.

Your panels and controllers

Success

You should be considering a separate charge controller for each 48V string of batteries, or at least one on the 600AH string and one on the two 800AH strings, assuming the two 800AH strings are composed of identical batteries.

The idea is to optimize the charging of each type of battery, which each charge controller can do for only one type at a time.

The charging is a separate issue from the harvesting of energy from the solar panels...that is a different set of parameters that are sometimes at odds with what the batteries need. However, a previous poster is correct--you want a MPPT (Maximum power point) controller that holds the voltage and amperage from the panels at the maximum power the panels can deliver. The controller will change the voltage from whatever the panels deliver, (usually, down) to the voltage the battery bank requires with very little loss.

As it sounds like you already have the control equipment, you would be best off to contact the manufacturer of the equipmen to get their recommendations about charging connections to battery strings of differing AH capacities.

Jon.

I'm not an expert, but I think parallel is inadvisable. It could lead to unequal charging currents for the individual cells.

You are right, most manufacturers do not recommend it at all. Even the banks he has now are not optimal and could easily result in shorter life spans of individual batteries as weaker cells get damaged.

To the OP:-

The absolute best way to charge (but physically impossible on most modern batteries) is to charge each CELL as an individual.....also very expensive!

But even having a 12 volt (nominal) charger for each separate 12v battery would increase the life substantially of all.

Batteries always fail first at the weakest cell due to reverse polarisation of the cell when it is the first one discharged. This applies to all battery types that I know of. Therefore its best NOT to take a battery down to the minimum, even leisure batteries live longer when handled well....

Also NOT charging LA batteries to 100% charge increases their life dramatically too, I work with max 70%, that is before gassing starts for most LA batteries.....Also I don't discharge below 12.5 volts as sulfating starts around that time. Which is why when a battery goes below this level it should be recharged as soon as possible.

See the manufacturer's literature.

Whether this applies or not to other battery types I cannot say for sure, but if ANY battery gets warm, that is usually a bad thing!!!! So don't charge too fast or for too long!!

Leisure batteries are more forgiving of deep discharges....but also more expensive.

This sounds like a scaled-up version of the problem with which every electric model aircraft fan is familiar, of how to charge series and parallel connected lithium cells rapidly. In that world the battery chargers now contain balancers, so that no individual cell is overcharged. The same idea is possible for charging in parallel. I have found this firm which claims to make a device to do your job of balancing parallel 12 V batteries:

http://www.cbcdesign.co.uk/batteq.html

though I have no personal experience of the product.

Agreed, that was a great talk. It will be interesting to see if/when/how it develops in the marketplace.

Hi,

As for my experience, the AH rating rating should not matter to the charge controller.

Since the V is same [48 Volts] and AH are different for battery banks, the single charge controller can take care of charging/discharging the battery banks.

Here is an example:

If 800Ah bank requires a constant current charge of 15A and 600AH bank needs 10A, than Charge controller will decides how ampere to push in to battery banks.

Remember since u have battery bank, not all banks are charged/discharged once.

It depends on priority you have set.

Thanks

Ravindra.R

Dear All,

Find reply I received from Alan Fidler
CBC Design

There isn't much of an implication for the charging system but unfortunately there is for the batteries!
It is not possible to parallel up sets of differing capacities. The reason for this is that the cells in parallel try to maintain a balance with the net result that one set will overcharge whilst the other undercharges. The result will be a wild imbalance between the uneven capacity cells.

Only cells packs of equal capacity can be connected in parallel with other cells packs.

Regards
ASAD

I'm sure your expert is correct, but it would be nice if he could explain the reasoning behind his words of wisdom. He is, after all, in the business of selling balancers, and the advice might be slanted in the direction of selling more of them. I appreciate that when a discharged cell is paralleled with a charged cell, then there will be briefly a very high current which probably does no good at all, but when they are permanently tied in parallel that situation does not apply. The equivalent hydraulic model is filling two water tanks, with an interconnecting pipe, from a single tap.
Incidentally, paralleling a discharged cell with a full cell is exactly what happens when one uses jump leads to start a car with a flat battery.

You would compare a parallel connection made for less than a minute to a 24/7 connection?

Not a good idea....

Also, generally cars have a single 12 volt battery, not 3 or more 12 volt Batteries in series......though this will change in the near future I am told to 48 volts (waitup till a new car jump starts an old one.......it will happen I am sure!)

You would compare a parallel connection made for less than a minute to a 24/7 connection?

I do indeed, my theory being that an intermittent parallel connection is much more dangerous to the batteries than a permanent one. You have to explain to me the error in my thinking.

You have to explain to me the error in my thinking.

Why must I improve your education? I am sure that nothing I say will change your flawed opinion...... So I won't bother....

Stay with this blog and I am sure you will learn a lot!!!

Have a great day.

....and all bought from the same manufacturer on the same day from the same manufacturing lot......

Many forget this!!

Asad wrote:

"Only cells packs of equal capacity can be connected in parallel with other cells packs".Regards ASAD

Why then did you bother post your question? You already knew the battery banks were not of equal size. There seems to be a disconnect in your logic. Common sense should have told you it would not work. Would you place wheels of different size on either side of a car? Especially one fitted with only one engine!

I am surprised that this group of experts did not give the most obvious explanation of all. Mechanics and electricians having any experience with multi battery banks will tell you that charging several batteries in parallel will result in premature failure of the bank as a whole, because one battery will fail within two years. This has been common knowledge for at least 20 years that I know of. At least that is when I first got told about it by an old mechanic.

The electrical explanation being that in any system of multiple parallel paths the current will divide according to Kirchovs law. This means one battery will charge faster than the others and the battery with the highest internal resistance will charge the slowest. Since only one charge controller is trying to feed two or more batteries it will respond to the voltage sensed in the highest level battery. If it is a smart controller it will alter the charge rate leaving the low battery insufficiently charged.

The end result is the weak battery fails first and prematurely. This in turn tends to affect the remainder of the batteries in the bank.

I used to work for one company where new batteries seldom lasted 18 months in service powering inverters because they were invariably paired up with old batteries. When we began replacing all batteries in a bank at the same time, duration between bank failure improved dramatically. With an entire fleet to maintain it was not difficult to find use for individual batteries instead of in banks powering inverters.

This is about as non technical an explanation as I can come up with. There is plenty of details regarding the electro-chemical activity involved but this tends to get somewhat tehcnical.

GA. Good explanation Elnav.