High current lead acid battery charger controller

[avz]

Hi
I have an electrical forklift that operates on 80V battery (40 cells). it has a charger but the control cicuit is beyond repair, the transformer and the 4 diodes bridge rectifier are fine. I would like to find a circuit that will be able to determine when the battery is fully charged and then actuate a relay/contactor that will shut off the charging in order to prevent overcharge. could it be done and if so, how?
Thanx.

 

[MikeMl]

Sure. How about using an SSR to switch the primary of the big transformer? Turn on the charger when Vbat ≤ Vcutin; turn off the charger when Vbat ≥ Vcutout.

For Flooded-cell Lead-Acid Batteries at 70degF, Vcutout should be 40 cells *2.35V/cell = 94V.

When disconnected from a charger, the resting voltage of a cell will slowly settle to near 2.10V/cell, or 84V for the string. I would use ~2.15V/cell to trigger onset of charging, which makes Vcutin = 86V.

I have a simple circuit consisting of a TL431 regulator used as a reference, and a 555 timer used as a window comparator which can drive an SSR, but would have to tailor it to your Vcutin and Vcutout. The circuit has been in use for several years on a 24V scissor lift.

Does that sound like it would work for you?

 

[avz]

Hi MikeMl
That sounds great! it seems that it might be a perfect solution. will be great to get the circuit diagram and a short explanation about it.
thanx.

 

[MikeMl]

Hi MikeMl
That sounds great! it seems that it might be a perfect solution. will be great to get the circuit diagram and a short explanation about it.
thanx.

Here you go. The TLC555 is used as a window comparator. Fed from a regulated 12V from the TL431 makes the TLC555's switching points 4V and 8V, which correspond to CutIn=86V and CutOut=94V, respectively. A precision 1% voltage divider samples the battery voltage, and the voltage at its taps feed the comparators.

The TL431 is powered by the battery, and draws about 9mA through R3, which must be a 2W resistor. The LED in-series with the SSR input is not necessary if the SSR has one built-in. The Power switching side of the SSR is not shown. It should be rated 120V or 240Vac (depending on your country) at 40A.

 

[avz]

Thanx a lot Mike Ml. I'll collect the components and inform you after I'll build it. thanx again for your help.