# 250Voltage discharging turnoff circuit

I'm reconditioning my hybrid battery. To do so requires multiple complete discharges which I do not have time to be around for. To avoid damaging my battery I've been attampting to design a circuit that would step down load as the voltage of the battery drops and totally secure the voltage once the end point is reached. Starting voltage ~240 +/- 5 Vdc

I would like ~200-300W draw at 2-4amps till 190V

~100W till 170V

~50W till 168V

~25W till shut off Voltage.

Would it then be possible to have a selector for the shut off voltage of 17V/85V/134V depending so as to successively discharge deeper over multiple runs?

If not I'd like a shut off V of 85V.

• What is your main question -is it just the one about the selector? – Andy aka Feb 4 '16 at 10:03
• Part 1: There's an issue here since 4 amperes out of a 240 volt source is 960 watts, and 2 amperes comes out to 480 watts. Also, since power will vary as voltage drops it would be better, in my opinion, if you specified the maximum current out of the battery at each voltage step point. That way, a simple switched resistive load could be used at each point and, when the voltage fell to the new point, a new resistor could be switched in in order to limit the initial current to the new value. – EM Fields Feb 4 '16 at 10:08
• Part 2: It's possible, and easy, to set variable shutoff voltages. However, if it's imperative that fixed power be drained between each of the discharge points, then the entire project gets much more complicated and expensive. – EM Fields Feb 4 '16 at 10:19
• @EM fields You're right i guess a better goal would be abt a 2.5 amp for the first long discharge, then 1.5A, .75A, then .25A for the final discharge. Nothing really needs to be held super constant, and I expect a continual drop of voltage that accellerates near the end of the procedure which is why I want to step down the draw. – jing7873 Feb 4 '16 at 14:59

## 1 Answer

This is all possible . You can make any DC mask you want . Your power is rather low so you could just burn it up in resistors and current sinks .The volt thresholds are easily done with simple compariters like LM393.If you use mosfets for your current sink be careful .Mosfets in linear mode are for people who know what they are doing .Otherwise just use BJTs .You can put them in series if you like because your currents are low and you dont have to worry about current sharing.This has all been done by others and works .One thing that I did was use an interleaved buck convertor into some elements .The idea of this is that you can vary the effective resistance of the elements to make the desired VI curve.This worked well for 110V 10A with 1.75 Vpc UVLO for testing a 50 cell lead acid bettery.I interleaved by 4 using a LM339 and used 4 cheap to220 mosfets that ran cold on no heatsink .The elements got hot of course but I run a pwm reg to drive a computer fan which kept things cool .The interleaving or polyphasing was done to keep gross ripple low meaning that huge electros wernt needed .I am told that lead acid does not like ripple but I think your batteries dont mind .I did all this last century so you might want to use a micro .