Amp-hour meter?

[Newshound]

I have several 12V lead acid batteries of different sizes with various uses. It would be handy to be able to check their capacity, and to know when they are fully charged. Most of the commercial meters seem to be aimed at boats or solar panel systems, and therefore attract premium prices. The cheapest seem to be made for the ham radio or RC model community, but are still around 60 dollars. Does anyone know of anything a bit cheaper (or even that price, but from a UK source?). I'm happy to build one, but I don't really want to have to make the PCB.

[Dave Plowman (News)]

I have an ACT which cost a lot more than 60 dollars. They're quite a complicated piece of kit.

[Colin Stamp]

One cheap, dirty way is to use an analogue clock. Use the battery under test to power the clock through a simple potential divider. Set the clock to 12 o'clock, add a fixed load and then do something else for a few hours until the clock stops.

It's all a bit crap of course. It can't really be calibrated and it's hard on the batteries, but it would allow you to compare one battery with another.

Well, you did want cheap...

Cheers,

Colin.

[Theo Markettos]

There are various chips from the likes of Maxim, Analog Devices, TI, Linear Tech, etc etc that will do it: 'battery monitor', 'gas gauge', 'watt counter' etc. Look in Farnell. The chips are a few quid, but the cost saving is probably outweighed by the faff in interfacing to them (unless you have an Arduino or something lying around). The 'gas gauge' ones are designed to drive LEDs etc so might be simpler.

Theo

[Dave Liquorice]

Hydrometer.

[Dave Plowman (News)]

There are no little electronic gizmos that do the job properly.

Yes there are. My ACT unit does all that - in a device little bigger than a DVM. It measures the battery voltage, ambient temperature, then puts a very heavy load across it for a short defined time. Then calculates the actual amp.hr capacity. But it is not cheap.

[Dave Plowman (News)]

Not to measure the amp.hr, you don't. That is done at the 20 hour rate. However, if it passes this test it will also deliver its peak current capability. The two are interdependent.

[Alan Deane]

>> There are no little electronic gizmos that do the job properly. >

I also have an ACT battery tester - very good, but way above the OP's budget I suspect. There are a few on ebay, item 280804981670 for example.

[Newshound]

Not a lot of good on gel batteries though

[Newshound]

>>>>>>> There are no little electronic gizmos that do the job properly. >>

Thanks, I'll think about one of them. But this is a bit more than I wanted. For a car battery with a big cranking current, internal resistance makes a lot of difference and you need to test at high current to get an accurate reading. Either with an ACT or the "old fashioned" sparky sort where you put an electric fire element across it and measure the volts. But most applications are not like this.

I have a couple of leisure batteries which are used to run an inverter for lighting and sometimes a pump at a stables with no mains, some old car batteries which I use for electric fencers, another gel battery which drives a clippers. If I had an amp-hour meter then I could check the capacity of these when charging them from flat, it would help me know how long I will get at different loads, and also recognise when batteries are losing capacity. Also it would help me make sure I go to full charge without leaving them longer than necessary. I know that voltage gives an indication of charge, but you have to leave it a little while after you disconnect.

But thanks to all who responded

Steve

[cynic]

ebay HIGH CURRENT CAR BIKE BATTERY TESTER vehicle auto 6 12v =A319.99

[Dave Plowman (News)]

Indeed. But to do what you want and quickly, it's the only way.

The high load tester doesn't really tell you much about the battery capacity, though. I recently replaced a car battery which would start the car ok when fully charged, but was well down on capacity. The other way round, I'm told, can't exist.

Given the cost of replacing all these, a decent tester would seem like a bargain. However, as regards charging a smart charger won't hurt them regardless of how long left on.

[Newshound]

Thanks, but this is basically testing for bad cells on a battery which has to run a starter motor. My applications are all at currents of no more than a few amps.

S

[Newshound]

Except that it has that extra functionality, the short duration high current test which doesn't bother me too much.

Superficially, it seems to me that if you put a modest limit on the current handling capacity (say, a few amps) then it shouldn't require all that much electronics, given modern chips, to count amp-hours. It's basically an ammeter, a timer/sampler, a bit of memory, and a display. Only a little more "stuff" than in a basic DVM, and you can get quite a reasonable one for a tenner.

Whereas the "high current" test is a bit like an earth loop impedance tester, and you need protection to stop stuff from blowing up if the microprocessor hangs.

Point taken. I think most of my chargers are OK

[Dave Plowman (News)]

It's not an ultra high current test. The leads aren't thick enough for that. Just high in DVM terms.

Of course it could be a lot cheaper if made in vast quantities. But it's not. I dunno if the Chinese do a cheaper copy - but it is quite well protected by patents. Which, of course, also cost.

[Theo Markettos]

Chips to do this (voltmeter, ammeter, A-D, integrator, I2C output) are about

3 quid, just add a series resistor for the current measurement and a pile of capacitors to keep it stable. Then a microcontroller and a little LCD, and some software, job done.

Theo