Lead Acid Battery Charging

Lead Acid Battery Charging for high voltage battery packs, 24VDC, 36VDC, 48VDC, 60VDC, 72VDC, 84VDC, 96VDC, 108VDC, 120VDC, etc. If you have questions, ask Bryan Andersen.

Apparently it isn't then best idea to charge a 48VDC lead acid battery as one string of cells. Instead one should use separate, but stacked chargers to break the string of cells into at most 6 cell units. The reason this is done is to help prevent cells from getting reversed when deep discharges happen. When you have more than one cell in series, it's always possible that one sell will get charged to a greater voltage that the other. This is life. All manufactured objects have some level of imprecision in their manufacture.

A 48VDC lead acid battery has 24 cells, so this means 4 chargers, each handling 6 cells each. One does this by having the charger handling cells 7-12 have it's ground hooked to the positive of the charger handling cells 1-6. In the end it looks something like this:

                       +-----> 48VDC Nominal
                       |
             +---------+
+---------+ |         |
|       + |--+        ___
| Charger |         _______ 6 Cells in series
|   # 4 - |--+        ___
+---------+ |      _______ 12V Lead Acid Battery
             |         |
             +---------+
+---------+ |         |
|       + |--+        ___
| Charger |         _______ 6 Cells in series
|   # 3 - |--+        ___
+---------+ |      _______ 12V Lead Acid Battery
             |         |
             +---------+
+---------+ |         |
|       + |--+        ___
| Charger |         _______ 6 Cells in series
|   # 2 - |--+        ___
+---------+ |      _______ 12V Lead Acid Battery
             |         |
             +---------+
+---------+ |         |
|       + |--+        ___
| Charger |         _______ 6 Cells in series
|   # 1 - |--+        ___
+---------+ |      _______ 12V Lead Acid Battery
             |         |
             +---------+
                       |
                       +----> Ground

It also has another side effect. Each 12V charger is much easier to construct that a 48VDC charger is. There are chips designed for optimal charging of that size of battery. Also when dealing with high voltages, electronic circuits become expensive.

The chargers will need be isolated, but this is easy. If each charger is made with it's own transformer, then it's easy to provide the needed isolation. A standard transformer has it's primaries isolated from it's secondaries. This will do fine for providing the needed isolation. When constructing your chargers, don't connect the grounds for the primary side to the grounds for the secondary side, and you should be fine. You will need a separate transformer for each charger, but then you won't need quite as large of one either.

The unanimous reply to my query last week about single chip rechargers for gel cell batteries was to recommend the Unitrode UC3906. A construction article using this cell appears in the June 1987 issue of QST magazine, pages 26-29. Kits are listed as available from:

A & A Engineering
2521 West La Palma, Unit K
Anaheim, CA 92801
Tel: 1-714-952-2114
(From David Prutchi)

If you want to build a circuit that takes good care of the sealed lead- acid battery charger, you can use Unitrode's UC2906 or UC3906 "sealed lead- acid battery charger IC". Both the data sheet and one of their applications sheets will guide you in the design.

QST of June 1987 has an article on charging gelled-electrolyte batteries of all sizes:

"A New Chip for Charging Gelled-Electrolyte Batteries",
by Warren Dion (N1BBH),
Publushed by QST, pages 26-29,
June 1987.
It is a construction article based around Unitrode's UC3906 chip, and it also presents operational and charging considerations for these batteries.

From - Fri Sep 3 17:36:01 1999
From: "P. A. Cavaney" peter@666cavemen.demon.co.uk
Newsgroups: rec.boats.electronics,sci.electronics.design,sci.electronics.misc
Subject: Re: Backup supply circuit
Date: Wed, 25 Aug 1999 01:10:14 +0100
Sender: "P. A. Cavaney" peter@cavemen.demon.co.uk

>> Can anyone help me with a circuit for a 12volt supply with backup. I.E. 12
>> volt supply and trickle charge to a sealed lead/acid battery so that the
>> battery will take over if the mains fails.
>>
>> Tia Gerald

Assuming you do not want to supply a large load, a simple circuit I have used for years in alarms and such is:

12V regulator with 2V4 zener in the ground leg - this jacks the output up to 14.4V. Connect to this output via a 3A diode like a 1N5402 or similar, which has a 0.7V drop across it's junction - and so you get 13.7 volts, just right for float charging your 12V battery.

This won't melt your battery and will be reasonably overload proof. Remember to fuse everything.

--
P. A. Cavaney

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