Repairing a lead acid battery charger

[Chris Jones]

Since its been in use it may well be serviceable or repairable.

There are 3 ways to desulphate it:
1. Charge it from the mains - via a load of course! and rectifier
2. Charge it with high i inductive pulses
3. unseal it & add chemical tablet

Given the cost of the thing I'd try to charge it and desulphate it
before condemning it.

The final option is to rebuild it, which might even be worth it for
£500.


NT

For what it's worth, I've successfully reanimated a sealed lead acid battery
that was kept totally discharged for maybe a year (zero volts), at least
it's good enough for what I need it for anyway.

To begin with, the battery was high impedance and wouldn't accept any
significant charging current at the proper voltage. What I did is connect
it to a current limited laboratory supply, initially set to a low current
limit (100mA for a 7AH battery) and I turned the voltage up to about 8
volts per cell. The low current limit is to prevent too much heat from
being generated in the battery due to the high voltage. The battery passed
only a very small current at first, in spite of the very high charging
voltage.

After some hours, the current built up until the current limit was reached
and the voltage gradually dropped down to the normal charging voltage, and
I was able to put an amp or two into the battery, at which point I set the
voltage limit to the rated float charging voltage printed on the battery.
I waited a couple of days till the battery had accepted a charge, then I
tried it out under heavy load, and it has been satisfactory ever since.

Another time I was unsuccessful in reviving a battery because it developed a
shorted cell either before, during or after the revival procedure. Since
it was given to me as dead, it is quite possible that it had the shorted
cell before it was allowed to get deep-discharged.

Chris

 

[ehsjr]

No, you're right, it hasn't been on float charge. During its life, the
battery was used on 15-20 occasions for periods of about 10mins only.

Thats about 2 1/2 times per year. If it was charged properly
after each use, then there may be hope for it.

I tried to charge it with the 1A charger but, as someone has pointed
out, the charger had to be left on for long periods to make an
impression on the battery voltage.

The open voltage of the old battery is now at 1.1V. It's a sealed lead
acid type so I can't do anything chemically to it.

Do I have to buy a high capacity float charger if I want to keep the
replacement battery alive?

Yes, if you replace it with an identical or equivalent battery,
you need a charger with greater capacity, even for float charging.

Better still is to review what you are doing with the battery
and perhaps come up with something different. That's an
expensive battery - and you might be able to do whatever it is
you are doing for a whole lot less money.

Ed

 

[Pandora]

I successfully repaired the battery charger!

I then decided to see if I could test my old (presumed "dead") 2V
battery with it.

The open circuit voltage of the battery was about one volt. I
connected it up to the battery charger and switched on at the "2V"
setting. The charge current was a steady 1A but I now measured the
voltage across the battery terminals at 0.5V and falling (the voltage
fell below 0.4V before I switched off).

I changed the range to "12V" and switched on again. The charging
current fell to 280mA and the terminal voltage was measured at 0.67V.
The terminal voltage is still falling but quite slowly (<0.01V per
minute) now.

Does this suggest that the battery can still be saved?

 

[Chris Jones]

I successfully repaired the battery charger!

I then decided to see if I could test my old (presumed "dead") 2V
battery with it.

The open circuit voltage of the battery was about one volt. I
connected it up to the battery charger and switched on at the "2V"
setting. The charge current was a steady 1A but I now measured the
voltage across the battery terminals at 0.5V and falling (the voltage
fell below 0.4V before I switched off).

I changed the range to "12V" and switched on again. The charging
current fell to 280mA and the terminal voltage was measured at 0.67V.
The terminal voltage is still falling but quite slowly (<0.01V per
minute) now.

Does this suggest that the battery can still be saved?

If the voltage of the battery under charge is going below the open-circuit
voltage of the battery, then it suggests that you have connected the
battery charger backwards, or the charger is still defective. If this is
what happened, then of course stop it ASAP as it will tend to kill the
battery.

(The only other possibility that I can think of is if the battery had
previously been charged backwards, then its terminal voltage would be
negative (i.e. the wrong polarity compared to the markings on the battery)
and would go towards zero when the battery is charged with the correct
polarity.)

Chris