This is why battery meters are a joke.

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This is why battery meters are a joke.

Postby Burgerman » 17 Aug 2022, 13:24

There are many things, such as the surface charge effect peukert and internal resistance meaning voltages change and drop or rise over time or fall chemicaly under load.

But the also RISE frequently as you take power (Ah OUT) of the battery...

So here I am taking 4A continually over three quarters of an hour. While the voltage initially fell fast, and the battery guage told me I had lost 3 lights... Then it rose over time as we DISCHARGED the battery. So now I took 3Ah out and the battery gauge is tlling me its now fully charged again...

Volts do NOT = state of charge even on a simple lead battery! Or they would be falling, not climbing here:


Image1.jpg



Whats really happening is that the battery has an internal resistance, and so as you load it up slightly it warms up. And all chemical reaction work faster as temp increases. This may only be fractions of a degree. But it means higher terminal voltages and a lower internal resistance. So the voltage increases as you use up the battery initially. But the battery meter doesnt know this. And it can never know the core temperature or figure out the difference betwen voltage drop due to discharge, or load or temp changes, or pukert, or previous 24 hour surface charge history... So they are useless. They are just feelgood devices to humor the driver.
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Re: This is why battery meters are a joke.

Postby LROBBINS » 17 Aug 2022, 14:09

The only way to keep track of remaining charge is by measuring AmpHrs used, but that requires being able to integrate Amp draw over time. There are add ons that can do this, but, if using a multi-cell battery, will add a constant drain on cell1 or 1 & 2 so cause an imbalance. In my CANbus system I integrate the Roboteq battery amps as an estimate. Not ideal because some things that use current are not fed by the Roboteq - seat actuators, brakes, and Rachi's computer, so simply the AmpHrs passed through the Roboteq is an underestimate of actual use. To roughly compensate for this, my computer says 1/2 amp-hr (500 mAHr) has been used when the Roboteq says 390 mAHr have passed through the Roboteq. With experience I'll probably be able to adjust that 390 for a bit more accuracy, but for now it's good enough. If the bar drops to yellow, I know that we've used about 1/3 of capacity (or somewhat less) and if it drops to the red we've used about 2/3 of capacity and I should think about re-charging. If it's taken 2 weeks to reach that point, I know that I have about another week at that usage level before getting into dangerous territory. In other words, when the bar touches the red it's well short of damaging the 200AHr LiFePO4 cells (and I do have a low voltage inhibit as well in case I push things too far).
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Re: This is why battery meters are a joke.

Postby Burgerman » 17 Aug 2022, 16:38

That will work OK for you. Esp with lithium where you can go a week without charging.
The above battery is lead though.
But with lead even measuring Ah out doesnt tell you much. Because if you use the battery hard, and do say 2 hours at 6mph then you are out of range. Chair stops. And you used 35Ah from a 70Ah battery.
If you did that over a day, you might get 55Ah from a 70Ah battery. And that also depends on the actual voltage drop, which depends on load. That may be high in a morning and low in an afternoon. So you will go further. Or the other way around and you stop much sooner. Its easy to run out of battery sooner if you climb a hill which drops the voltage chemically (surface charge effect). This happens faster on a gel than on AGM because the electrolyte is immobilised and gets weak right next to the lead. That takes up to a day to fully recover.
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Re: This is why battery meters are a joke.

Postby Burgerman » 17 Aug 2022, 20:27

Battery recharged again. This time I charged it fully the same way. And test again... With the air conditioning blowing onto it on my bench. And after waiting for 3 hours after charging. For the surface charge to soak into the battery plates pasted lead interior.

COMPLETELY different result. Same discharge rate Same settings. Same battery.
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Re: This is why battery meters are a joke.

Postby ex-Gooserider » 23 Aug 2022, 02:38

What has always amused me is the way the batteries appear to magically recharge themselves while you are sitting still.... I've had my Pilot+ joystick showing down to the yellows as I'm running around and had been for quite some time, but then I stop for a couple of minutes and it's back to showing 'fully charged' w/ all the greens lit again...

I know why it's happening and pretty much ignore the gauge other than being more likely to plug it into my van for a top up while driving if I've seen more than one light go out at all...

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