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I understand that 12.2v is considered about half full, 11.8v is considered empty, but that's without load when the battery is at rest for a while.

Is there a way to determine half full when under a specific load, ie 20 amps and 35 amps?

The battery is an aging 100ah flooded lead acid.

In a test today, after being charged on a 7 stage charger, it dropped immediately from 12.6v without load, to 11.64v with a 19 amp load. Then took 30 minutes to drop a further .64v to 11.0v.

Does that look like the normal behaviour of a healthy battery, and how do I tell how low I can go when its under a 20 amp, and a 35 amp load?

I've posted this question on different locations around the globe as it's the first thing that the Internet seems to want to keep secret in 25 years of searching for stuff.

I hope I haven't broken it:)

Mr B

See the attached table (you can google "12v battery voltage charge" for similar charts)





12.6V = 90% charged for a Lead Acid battery

dropping from 12.6 to 11.0v means that 90% of battery capacity has been used.

for this battery you should get ~ 12V X 100Ah X 90% = 1000Wh

of that 1000Wh, often up to 50% is wasted as heat, i,.e. ~ 500Wh usefully energy should be available.



You have used the battery for 30 minutes at 19 A, i.e

work extracted from battery is 12V X 19Amp X 0.5h = 100Wh



Based on this it seems that the battery is "broken"!

Note - If you discharge a lead acid battery to below 11V you have broken it. Lead Acid batteries should ideally not be discharged to less than 80% of full charge - i.e. do not discharge below 12.5V


Also note - running a 100Ah, 12V lead acid battery at 19A is a sure way of killing it. I would recommend not drawing more than 5A from a 100Ah battery for more than 10 minutes.

There is a lot of information on the net on getting the most out of a lead acid battery.

Gabe

So confused.

Some things I thought were fact.

A deep cycle lead acid battery will float at around 12.6v when it's new.
11.8v is empty, but this is when measured without load, and after the battery has been sitting without load for 6 hours or more. So 12.5v would be almost full if there was no load, and full as a crazy full thing if there was a load.

If you get a new 100 ah battery out of the box and put a 20a load on it, what will the voltage show?

How can a person run an eBike or a trolling motor if we cant go below12.5v?

We bought eBike front wheel conversion kits.

Everyone go out and buy eBikes. They are awesome!

But...

Our eBikes have 10ah 36v batteries. The motor is 250w and everything cuts off when the controller says we have used too much battery. That's roughly an hour at full throttle and the batteries come with a 1 years guarantee. I cant see how they can offer that if what you say is correct.

SO TOTALLY CONFUSED!!!!!!!!!!!

I thought a 100 ah rating was 100ah at 20 amps draw. C20 or something

ARGGGHHHhhh!

I need either a new brain, or at the very least, I want my old brain back.

Your ebike probably have a lifepo battery that can handle a lot higher discharge current as well as been able to discharge deeper without damage. Google lifepo vs. lead acid battery for the differences.

No, its definitely three lead acid 12v batteries to make the 36v system.

Note to anyone buying an eBike, lead acid batteries are really, really heavy. Avoid them if your budget allows.

Please keep the input coming as I'm feeling a little lost out here on my own.

If I seem to be questioning you it's only because I thought the universe was a little different.

Pesky universe.

WHY DIDN'T I PAY ATTENTION IN SCHOOL?

A key point here which is probably whats confusing you is that those Lead acid voltages are "resting", that is left to equalize for many hours with no load.
Firstly volts is a very rough way to check capacity BUT its easy and cheap to measure.
Problem comes where control or monitoring needs to be done instantly and under load soooooo....factors and offsets come into it. Means many voltage based control setups are pretty rough. Problem is that its all we have in many cases.

Entirely dependent on what sort of battery it is- flooded cells, AGM, gel, deep cycle, auto starting, LiFePO4, or other Lithium variety, etc




You can go below 12.5V!




20C would be 2000A, C/20 would be 5A, C/5 (or C0.2) would be 20A

The 100AH rating will be for a specific discharge rate, probably C/20 or 5A. At higher discharge rates you will get a lot less than 100AH from it, due to the Peukert Effect.

This may be also be useful.



If you have a new 100ah lead acid battery, and you drain 100% of the capacity from the battery over a 20h period (~ at 5 amp), you can expect a useful life of 500 cycles.


For a 100ah battery at discharge rate of 20 amp (i..e drain in 5h), you can reduce the useful life at least by a factor of 4,

If you drain the same new battery in a very short time (i.e. short the battery) - your useful life will be one cycle!

Gabe

I ran a stress test of my solar system and [url=
http://120thingsin20years.blogspot.com. ... cycle.html]got these numbers[/url] ...


they dont display here but they look like this when I try...


Minutes
1
30
60
90
Amp Hours
less than 1
8.71
16.14
23.29
Volts under load
11.7
11.44
10.94
10.36
Volts after 1 minute rest
12.4
12.22
12.06
11.9

After the test I waited an hour and the voltage had bounced back to 12.31v.

I turned it on for another 10 minutes and the voltage dropped rapidly to where it was before the hour long rest.

Final numbers after the additional 10 minutes were 10.36v under load, 12v after a minute rest, and 26.54 amp hours used.

Given all that and the fact that the voltage bounced back to 12.31v after an hours rest, I think that means battery might be ok, and that I can run it down to at least 10.5v under a 16 amp load and still be at 1/4 full.