Small SLA batteries

I have been a supporter of these batteries believing them to be good value. The low current ‘alarm’ battery is not an expensive item and if used for low current applications should be ideal.
On occasion I have had a customer complain that his battery has failed and they assure me they did not leave it discharged without recharging it.
In order to check the battery I take it in to my supplier and request they check it. It’s a 7Ah battery.
This is the response I get:

Hi Richard,
We have tested your battery.
On a 200W load the battery ran for 2 minutes.

You may collect your battery when you are able.

Oh jeez. Presumably that battery is now toast?

Thanks! This was my response as well. I have been told these low current batteries are made with recycled lead and cannot handle the high current (hence the reduced price!)

A 200W load on a 7Ah 12V battery will draw more than 16A, which is more than a 2C discharge. I wouldn’t trust any lead-acid battery to do that for very long, especially not a little gel battery.

They probably didn’t want to wait long to safely discharge it at 0.1C and observe the voltage drop over a two hour period of something…

You know, if you want something done right, you often have to do it yourself. Or so goes the saying.

As a fellow DIYer I often find that I have a lack of proper testing loads. Though in this case I would have likely used a 5W LED downlight that I have around (an old MR16) which is about a 5W lamp, or a 400mA load. For no other reason than because that is what I have. That’s about a C/20 (ish) load, so if the voltage is still good after 10 hours or so… the battery is far from dead.


This company imports these batteries by the container.(and not by airfreight either!)
They obviously have a problem checking batteries since it’s time consuming. However I was requesting information that isn’t easy to ascertain. If I knew how to determine the condition of a battery (besides it’s capacity and a dead cell) I would do it.
So I approached the supplier and the attitude I experienced resulted in a heated discussion and me escalating the matter and then having a chat with the boss. But we are no further in resolving the issue…
If one could open the battery and see sulphation on the plates then that’s a good start…

Please ask your supplier to replace the battery and not to damage it next time by running tests that draw more current from the battery than it is rated for.

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Need to stick to basics here. The battery manufacturer will give the AH rating of the battery depending on the discharge current. For example the Forbatt (reasonably well known locally available battery) 7.2AH battery has a 7.2AH rating at a discharge rate of C/20. If you discharge at a higher current then the capacity declines. See FB7.2-12.pdf (

The best way to test the capacity of a battery is with a tester that measures the AH discharged and to use a constant current discharge, in this case, of C/20 and measure the capacity when battery reaches commonly accepted end voltage of 10.5V.

As can be seen in the pic below, I used a capacity tester to test a 12V 200AH lead acid battery. It tested at 120AH using discharge rate of C/13. (Was too lazy to find exact load to test at C/10). The batteries are just over a year old and were not abused.

I tested a brand new 7.2AH Forbatt @C/20 till 10.5V and it had capacity of 6.8AH…

But there’s also safe discharge currents. My understanding is that the battery produces excess heat if you discharge it too fast (or charge) and that heat will end up damaging the cell. So yes, you can discharge your LA battery at a high rate, it’ll just lose some of its capacity in the future (or a lot of it).

What you should really monitor is the effective (what loads it can power, not including the losses due to heat generated) kWh you can deliver with a battery over its life, given a certain discharge and charge rate. Not what it can do in a single cycle.

Monitor, yes. But sometimes you need to do a diagnostic test, and then you need to be somewhat scientific about it. Use either C/20 or C/10 discharge rate (depending on whether you are in a hurry!) and measure using some kind of meter.

Or, just a known load and let it run for some hours, because quite often with these small batteries, we are well aware that they are degraded… we want to know how badly degraded they are :slight_smile:

Car batteries are really quite similar. I know mine is degraded. What I care about is whether it can still start the car three or four times on a cold morning. I refuse to replace it before it leaves me stranded. That’s the rules for myself. The rules for my wife’s car are different :slight_smile:

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Just park on a hill! What is this “battery”/“start my car” nonsense.

Parking lots don’t have many slopes so this can be tricky…

Man, I can tell you stories. We had this tractor, a Massey Fergusson 265. It was terribly hard to start on a cold morning. I mean, it would be 11AM and you’d still be unable to start that thing even with a good battery. But we also had another tractor, a David Brown from 1965. That thing starts every morning no matter the temperature even without any trace of a cold-start system.

So on cold mornings, we’d pull-start the Massey by towing it with the other tractor. But that meant some preparation was required the previous evening, 1) parking it in reverse, 2) unhooking any implement that was attached to it, cause you cannot move the thing when you have a ripper attached and the hydraulics don’t work!

Edit: Sadly my dad sold the Massey a few years back. And the David Brown? He was going to turn that into a big mower (because you can turn the crown gear around in the rear differential, and make it run backwards, with the hydraulics in front), but when he was robbed in a business transaction about a year ago, this particular vehicle was taken as part of the loot.

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So a 7.2Ah battery, brand spanking new and in perfectly good health does not necessarily mean it can and will deliver 1 amp for 7,2 hours?

This is fascinating thanks.

So I had better take a look at the battery and its recommended discharge rate and decide from thereon whether a particular battery would be well suited for its intended application (say for something where I know that the demanded amps will not exceed the battery manufacturer discharge rate specification)?

Probably depends on what voltage you are prepared to take it to…

Oh yes this is true. Didn’t think of that, thanks.

The ideal application for a lead acid battery is for an automotive engine starter. It has to deliver a hellava current for a second or 2 and then it sits there being charged for as long as the engine runs.
They will be around for as long as the internal combustion engine is (but no longer than that!)