Amp-Hour Tolerance

6.82Ah/7Ah = 97%, though note that the divisor is only 1 significant figure.

So?

The amp-hours you can get out of a battery depend - among other things - on the discharge rate, the temperature, and what you consider to be the end point. I'm sure a combination of these variables could be found which would bring the result up by the odd few percent needed to make 7Ah.

Thanks Guys.

Yea, for me I can say "what's the big deal", but the customer is 'king', so I need to find some data.

Unfortunately for me, he has found another battery, also 'listed' at 7AH, but measures more than 8 ... now he wants to SCRAP 1200 batteries and get the new one.

I can fight the issue, but was hoping there was some industry standard 'tolerance' that might help me 'convince' him.

Kind regards ...

Look in the catalogues of other manufacturers In general I found the AH value as a couple of values with a lower and higher limit. As far as I remember the differences were with respect to the mean value around 5..7%.

I hate to sound like a "lawyer", but, what does the language in the contract you have with each other says with respect to minimum standards on components.? Did they approve the brand and model of the battery you supplied? Scrapping 1200 batteries sounds like a real over reaction.

Well, to sound 'legalistic' is never wrong when the one you are combating is the same.

It's not a matter of contracts, but preserving relationships ... a little give, a little take.

They're a good customer so we want to keep them happy if possible. They have been a little (wait ... make that a LOT) like shooting a moving target, but, they are not totally unreasonable IF I can find some basis for a tolerance.

The problem stemmed from their product not having QUITE the battery life they expected ... then they hired a consultant who told them the battery was substandard. We're no battery experts, so we bought the same piece of test equipment they have and are doing our own tests, which is why I know the AH measurements are as I said.

It will be a debate, but not a battle ... just trying to learn more.

Thanks, and kind regards ...

I understand completely about "preserving relationships".

Next steps... if your shoes were on my feet, I would get ahold of UL and see exactly what the listing on their battery covers. The AH rating +- valuve may be in that document.

Are we assuming the customers meter is accurate?

Repeated 100% discharge will shorten the relative life of either battery and AH shouldn't be used as a go/no go gauge upon which battery selection is made.

This will give some note of worth

A larger (bigger or higher Amp hour capacity) battery with the same open circuit voltage will make your vehicle faster only if it has lower internal series resistance and can, therefore, deliver more current to the same resistance load. e.g. your motor.

Think about it using a bucket of water analogy. A fixed diameter hole (resistance), a fixed depth (voltage) below the surface, will leak water at the same rate (current) for any diameter(e.g. capacity) bucket.

It seems that your customers requirements for a 7AH battery for 1200 units does not have enough margin. Measuring 6.82 AH and claiming NOT enough is splitting hairs.

If the customer requirement is that the system be supported with a minimum of 7AH for 1200 units over the life of the system, a starting point for battery AH rating selection would be 7 * .2 (20% margin) or 8.4AH.

Batteries also have different AH rating at different discharge rates. Typically, a higher discharge rate reduces the AH capacity. In this case, knowing the load profile (typical current) of the system this battery is connected to should be considered.

In retrospect, the requirement for a specific AH rating for a battery leaves much to be considered. The requirement should read something like "The system shall provide x minutes of run time when in battery back up mode". Then considerations for duty cycle come into play (battery recovery-charge, etc) and have to be designed in; charge time discharge time, etc.

My haunch is bad jU jU on the part of the customer for calling out a 7AH battery.

Hello Del:

Battery ratings are anything but 'straight' forward, as you noted, they are very dependent on many factors. That's what all those nice curves are for, to give you some idea of what to expect. I went around and around with a couple of jokers who claimed that the manufacturer's own statements on battery charge/discharge rates didn't matter and got into some rather derogatory, childish name calling. I just left the worthless conversation. There should be some standards on measurements, probably in the ANSI and UL standards documentation but get out your wallet since it isn't free unless your local library has copies. Some manufacturers do reference particular standards on their data sheets but not all of them do it.

Hello "guest",

Truly, thanks for this information. I 'believe' the test system we are now using is based on this standard, but I need to check further.

BTW, I hope you take a moment to 'join' CR4 ... you have a lot to offer.

Thanks, and kind regards ...

Hi DCad,

we buy and sell a lot of batteries and I am sure I have seen Ah ratings referenced to a standard. I've looked through my reference catalogues but cannot find it now however in my search i found this , which you might find useful. http://www.mpoweruk.com/standards.htm

Best of luck, and come back with your results,

regards

Chas