Electronic Circuits and Projects Forum

I understand as the voltage drops the load would have to change also in order to keep a constant amperage draw.
In reality there is very little voltage difference between a fully charged 6 volt battery (6.35) and a completely discharged one (5.25).
I practice I would probably not completely discharge the battery. It's very hard on the battery and very time consuming. Discharging it to 50% (6.1)or perhaps a most 80% (5.99)would be suitable and come close to matching the actual performance expectations.
I used the formula R=E/I and determined that to produce a 25 amp load with 6.35 volts would require a resistance of .254 ohms. The difference in resistance between a fully charged battery and a completely discharged one is only. .04 ohms. Help me here if I'm going off the deep end. Purchasing a 155 watt resistor to that close of tolerance would be difficult. So I would no doubt have to do a bit of compromising. The load isn't really that critical with a little math I can compensate and what is more important is a comparison from one test cycle to the next.
So for now unless someone has a better idea let's go with a compromise on the load and except the fact that it will only show a mean value.
What is more important to this project is an accurate LVD it has to be accurate. The more accurate the better.
Thanks for your input!

I would stay with a 25 amp current regulator. Trying to use just a load resister would be inaccurate.
IE It mite be fine for the 1st battery but may be off for the 2 nd battery test do to the change in resistance after heating.

Not sure how close you need to measure them but it might be easier to manipulate the time rather than the load. For example: The 50% point using a .25 ohm resistor would be 25.44 amps at 6.36 volts(100%) and 24.4 amps at 6.1 volts (50%). But having said that I think battery voltage as a function of capacity is under no load. Maybe you can confirm that. It seems to be hard to find a LVD for 6 volt batteries - especially adjustable ones. This would be pretty easy to build but hard to find off the shelf.

Good point!
It would not be unusual to have 16 batteries to test.
Current Regulator?!? The water getting deep here. What such an animal look like and what kind of expense are we talking about?
I would actually prefer the LVD to be fixed. Or at best have three fixed values 6.10, 5.99 and 5.25 volts
I'm not shy about having to build a circuit but have not idea how to design one.

ronv
I just read the other part of your post.
Your right SOC is measured OCV but a test such as this measure the time it take for a battery to go from fully charged to the cut off threshold under the prescribed load.

Maybe somthing like this for a LVD
http://www.toadzrc.com/novak-smart-s...batt-5472.html

Okay, so all that leaves us with is the timer. I noticed you said self powered. Does that mean plug into the wall or having its own battery?

The timer is quite possibly the easiest part of the project
Self power because it can't depend upon power from the battery being tested.
Even with my limited knowledge of electronics I can envision several ways to do this.
Perhaps the simplest would be a Hobbs meter attached to a relay power by the LVD.

I viewed the LVD disconnect suggest but I'm afraid the cut out voltage is too high.
6.25 volts which represents about an 80%SOC
Also I'm not sure how the throttling function would be dealt with.

A circuit design for a LVD that could be modified to have three separate values would be great.

What about the current regulator?
Is this some thing that need to be built or purchase?