# Electronic Circuits and Projects Forum

## Internal resistance of a charging battery

1. Well, I did some PSpice-ing and came up with a number. I went back to my test setup and got some better values (the values in my original post were recalled from memory.... a very poor memory). The battery was actually at about 15.3V at the point we considered it "depleted". When power was turned on, the DC output was 21.5V. I figured out the equivalent resistance of the load at 0.88Ohm. I set up a circuit with a single battery, a 21.5V source, and the 0.88Ohm load. I tweaked the internal battery resistance until the currents looked pretty close to what I was measuring. The internal resistance came out to be 0.24Ohm.

I then added a second battery with exactly the same properties. This time I adjusted the DC voltage of the supply until the supply current was 50A. The end result was 18.85V. The attached screen capture is the final circuit that I think should approximately predict the numbers I was looking for.

2. OK, I understand your problem!

I think you are approaching the problem from the wrong side.
I think you need to look at the load characteristics of the PSU.
Load it up to 50A and beyond and measure the voltage.
Only then will you be able to determine voltage available, it is the PSU which is losing output volts, not the battery.

JimB

3. If a 24 volt SLA battery is at 13.5 volts it's beyond "depleted". About 21 volts should be your low voltage limit for discharge for normal lead-acid cells.

4. JimB,
I'm not entirely sure I follow you. When I turned on power to my test setup (one battery and the load in parallel), the DC PSU immediately pegged at 50A. So it was loaded as much as it could go. In order to keep the current at 50A, it has to lower its output voltage to abide by Ohm's Law. And if I put a 2nd depleted battery in the circuit, it'll sink even more current, which'll cause the PSU to drop its voltage even more. So I'm following you there, but I'm not sure what you're suggesting.

nsaspook,
Well, it was actually 15.3V, not 13.5 (see post #11 above) - although that doesn't make it much better. I'm intentionally creating a worst-case scenario. We were able to get the battery down to 15.3V in our test setup, so we have to assume the customer will also be able to run the batteries down that low.

5. Originally Posted by Rusttree
JimB,
nsaspook,
Well, it was actually 15.3V, not 13.5 (see post #11 above) - although that doesn't make it much better. I'm intentionally creating a worst-case scenario. We were able to get the battery down to 15.3V in our test setup, so we have to assume the customer will also be able to run the batteries down that low.
Sorry, mistyped the voltage.
Discharging it much below 20 volts with a load even once can damage the battery causing cell reversal and a internal electrical short. You need a low voltage disconnect long before 16 volts.

6. Yeah, that's actually going to be my primary recommendation: stop the batteries from ever getting so low in the first place. But that's for the powers-that-be to decide whether they want to make the hardware change.

7. Originally Posted by Rusttree
Yeah, that's actually going to be my primary recommendation: stop the batteries from ever getting so low in the first place. But that's for the powers-that-be to decide whether they want to make the hardware change.
At least put in an annoying (none silence-able ) warning signal.

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