12 Volt Battery Capacity Test

[jkinvt]

Hi

I have 2 MK  (ES 12-12) brand Sealed Lead Acid batteries for an electric bike,
they are rated at 12 volts and 12 Ah --- http://www.mkbattery.com
I'm trying to figure out how to measure how much capacity they
have after the charger light ---- signals they are fully charged.

I can't afford to buy an expensive load tester.

It appears, that if I wire 6 (#1157) 12 volt automotive bulbs together
and then use a Watts Up meter --- it may tell me the percent of charge
in the batteries (A few websites rate these bulbs at 2.1 amps each)

Is it possible to determine with reasonable accuracy the percentage of
charge that the battery has in it ?

Am I on the right track ? Is there a better way ?

Thanks for any information that you can offer.

Thanks   

Joe

PS My concern is heading out on the bike and running out of power a
half mile from home ---- as I (am sorry to say) that I am handicapped and
can't walk it home.  I'm trying avoid a potential problem.

[MJLorton]

Hello Joe,

Thanks for your post.

I'll post my thoughts on how best to test the battery capacity and see if others have some input / better advice.

Battery capacity varies due to several criteria...some of which are the rate of discharge and the ambient temperature. A higher the rate of discharge will reduce the (rated) capacity (Ah) available due to internal resistance and energy lost to heat.

So, before you setup to do your test you should try and find out what current your electric bike draws when in operation. This way you can run the capacity test at the same (or close to) current draw to get a more realistic result.

Once you know what the current draw is, you can use the WattsUp meter with the correct number of bulbs (for the test current required) connected in parallel. I would stop the test when the battery is at about 12.2-12.0v and then note the Ah capacity and the time taken to get there.

Do you have a voltmeter on your electric bike?

Cheers,
Martin.

[jkinvt]

Hi Martin

Thank you for the reply and info.

Here's what I know.

( The batteries (are MK ES12-12) and are marked ..... 12V, 12AH, 20HR )

There are 2 batteries wired together in the pack.

The motor on the bike is rated at 2600 RPM & 450 Watts, with an electronic controller, and

has a variable speed throttle.

(www.ridekick.com --- also You Tube --- www.youtube.com/watch?v=hRYk6uv2UeA

Found the below info on Google. I'm guessing this is what you meant by "rate of discharge".

"The capacity printed on a battery is usually the product of 20 hours multiplied by the constant current that a new battery can supply for 20 hours at 68 F° (20 C°), down to a specified terminal voltage per cell. A battery rated at 100 A·h will deliver 5 A over a 20-hour period at room temperature."

It appears then, that the electric motor (24 Volts & 450 Watts) draws 18.75 Amps (?) -----
(24 divided into 450 watts)

I’m a bit confused, at this point, it seems the info above may tell us what the batteries
are rated at and what amount of draw the electric motor is pulling --- I’m totally lost
on how this might relate, to the capacity (of charge ) the battery may have at any
given point ?  Am I missing something ?

No voltmeter on the electric unit, but I should be able to borrow one.

Please note --- you’re not dealing with the brightest bulb --- on the block !

Thanks for your assistance and patience

Joe

P.S. Currently minus 22F or minus 30C ---- so any outside testing will have to
be put off for bit.


[/b]

[dr_p]

Check this graph of a lead-acid battery:



If you just measure the voltage to assess the battery's "percentage left" it's not of much use, because the load current (probably) varies, and so does the slope of the discharge curve. A lead-acid battery that measures 12V can be either full or 90% empty, but with very low load.

What you need is to measure how many amp-hour you've drained out of it until that specific moment. With every discharge cycle you will get the hang of when the battery is empty. Or if you keep an eye on the Ah going in while charging, you know exactly when it will be empty.