How do you test your sealed lead acid battery?

[rogerg]

How do people test the charge of their sealed lead acid batteries? I'm after a simple indicator of percentage charge such that I can stop using it before I suck it dry causing damage.

The best I've been recommended is a multi-metre to measure the volts and get to know what's full and what's empty. I'm looking for a neater, more user friendly solution.

I use several different batteries, switching between them as required.

Thanks,
Roger.

[Starkler]

I have seen information that when a SLA batteries terminal volts are at 12V, half of its capacity has been used. Of course you should avoid running any lead acid battery flat. Low voltage cutouts for lead acid batteries are normally set at 10.5-10.8V to avoid damage.

[acropolite]

Goeff is correct around 12v no load is half discharged. SLA batteries rarely get charged correctly and rapid charging when dead flat can cause seriously reduced life. Also the life of the battery (number of charge discherge cycles) is relative to the depth of discharge, i.e. the shallower the discherge the greater the number of charge discharge cycles over it's service life. The most important thing is to fully recharge as soon as possible after discharge, preferably immediately or within a couple of days.

[RAJAH235]

Hello Roger, I built a battery voltage indicator, in 1998, using either an LM741 or LM 311, 8 pin chips. (Note; The 311 is completely different). I have attached the 741 circuit. Can be put on a very small veroboard. I have one mounted near my 12 volt, 7 A/H battery. I have it adjusted to ~10.5 volts to enable the "flashing green LED". ( make sure you use an accurate DMM, as the circuit is sensitive down to 0.01 volts).
HTH. :D L.
ps. Any gratuities gratefully accepted.......

[cjmarsh81]

Yes, the low battery cut-outs I have seen turn off around 10.5V. I built one of those **** Smith kits a couple years ago and it works fine for me. You can adjust a pot that varies the cut-off voltage for whatever you want. I also changed the relay for a mosfet so it is sold-state.

This does not tell you the battery condition at a glance though. You can build a battery level meter with a quad comparator chip. I use one of those on a 12V Lead Acid battery. It has a four led bar graph and I just look at it to see the state of charge. Very easy.

Doh, RAJAH got in before I finished typing. Yes I agree, make sure your DMM is accurate. You don't want to adjust it to turn off at 10.5V when it is actually 8V or something like that.

[beren]

Not sure if this helps but I just use a battery box that has terminals and a gauge already to go , you can find these at Supercheap

[stinky]

Not a good idea to go below 12 volts. You'll find that with continual deep discharges the capacity will reduce. Not a good idea to discharge a 'normal' sla below 50%.

[cjmarsh81]

Beren. Is this for a 12V 7AH battery? Mine is currently in one of those bag's that Jaycar sell with a cigarette lighter socket in the side. A box with a meter would be handy.

[beren]

I have a 35AH lead sealed battry and theres room to spare , it has one 12v outlet ...some pics

[cjmarsh81]

Ah ok. Little bit big for me to carry around.

[RAJAH235]

FWIW. The main reason I built this flashing LED indicator, was to give a visual warning without having to 'read' a meter or gauge. When down to the stated 10.5 volts, the LED flashes.
ptn. I was viewing one night before making this unit & my 'power on' LED went out. (only runs on 3 or 4 mA). Thought I had a bad connection or broken wire. Turned out that the battery was down to 4 Volts. Yes, 4 volts. Took home, charged up as per normal = no probs....Now I don't have to worry.
HTH. :D L.
ps. I have built a 'float charger' tho, so can leave battery on indefinitely.
pps. Total cost of indicator= <$6.00.

[Vermin]

I use this little widget from Jaycar. As well as monitoring the battery voltage you can monitor the ambient and one other temperature:

http://www1.jaycar.com.au/productView.asp?ID=XC0116

[rogerg]

G'day all, thanks for the replies.

The main battery I'm using is a 35 amp hr battery. So it should last a night, hopefully 2, but unsure. Until now it's just been powering the Losmandy which of course it could do for a whole day without a problem. But soon it will be powering the Losmandy, Toshiba with EOS350d connected (powered off it's own batteries), and maybe the SBIG eventually. I'm not sure how quickly it's going to get drained then (combined draw from all that is somewhat unpredictable depending on many things), so need to know the level.

cjmarsh81 - one of those kits sounds ideal. I'll check out what Altronics/Jaycar/DS have.

RAJAH235 - Thanks for all your info, but while I can solder wires and plugs, I have to admit that I don't remember enough of my university engineering to read the diagrams and understand the components, so it's a little above me :-)

Vermin - That gadget sounds great! (I'll take anything that glows and has a digital output, must be good). However one question - it says you can't turn the backlight off - is that so? is the light a problem? Temp too, that'd be neat.

Thanks for all the input, much appreciated.

Roger.

[Vermin]

The reasonably bright blue electroluminescent backlight was a problem untill I opened the case and snipped the wire to it. It's pretty obvious which one it is, but I can post a picture showing exactly which one if you like.

I'm also thinking of increasing the value of the series resistors that drive the two LEDs to dim them a bit. I'm also going to swap the green one for a red one. An alternative to doing this would be a blob of black paint or just cutting them out of the circuit altogether.

[rogerg]

Ahh, cutting the wires, of course. I once bought a weather station from Jaycar, had the most anoying high pitch beep every time you pressed any button. Snipping the wires on that worked too.

A push button or switch instead of a direct cut might be nice? ... Anyway, that's something I can manage.

Still not sure if I'll get this or a kit, I'll decide later.

Thanks,
Roger.

[bonox]

I run an inverter from mine in addition to the scope, and the inverter will certainly tell you when the battery potential falls too far! Piercing scream, laptop reverts to battery and the camera shutter clicks shut. Telescope keeps guiding though :)

[cjmarsh81]

That inverter has to be a serious power drain on your battery. Do you really need 240V?

[bonox]

i find that my laptop with its 4 year old battery doesn't run for more than 20 minutes at a time, especially in very low temps. Also don't have a supply for the camera that can be powered from anything other than a weird nikon proprietary plug, so on both counts i need 240v.

[Vermin]

For the computer check out the Targus range of 12V adapters, http://www.targus.com/AU/accessories_power.asp

I picked one up off ebay quite cheap, and I think it will be a bit more efficient than a general purpose inverter (though this is just conjecture on my part as I have no direct evidence).

[h0ughy]

I have one of those with a 110AH deep cycle sealded LA battery. excellent, easy to see whan you need a charge.

[ving]

got a problem with multimeters? I use one.... or my tongue :eek:

[rogerg]

I have one of those kind of things from K-Mart, it has a 17 amp hr battery in it. Problem is, the battery will die before the cheap indicator tells me it's empty. In fact, even when it's dead and unable to power the laptop the indicator stays on green.

Hence I have given up on it.

Roger.

[rogerg]

I just thought there might be a neater solution - single digital readout with no buttons or anything to get in the way. Thing with the multi-meter is getting it out, putting the pins on the terminals, turning it back to the right setting after using it for something else, and then turning it on. I thought maybe there'd be a nice solution I could just wire in to my plugs, wave there the whole time dimly illuminated or the like.. which, sure, I could do with the multi-metre too.... but..

Roger.

[ving]

I know roger, it such hard work :P

how about some sort of built battery level monitor between the battery and device?
sorry, i am not much help am i? :)

[RAJAH235]

Roger, my small veroboard unit is just 38 x 38 mm sq. & is screwed to my base as shown. It is wired directly across the battery/socket assy. Nothing to connect. It automatically gives an instant visual indication if the voltage falls below your set value. Very simple to make & also in operation.
HTH. :D L.
ps. It is waterproofed, naturally.

[cjmarsh81]

Rogerg, is the battery level meter real or a sticker? lol. If it came from K-Mart I would not be surprised. By the way, those 17Ah from Kmart are more like 10Ah. They use cheap batteries. I was looking at one myself and ended up going to battery world and getting a Projecta 17Ah. You can tell the better batteries by the weight.

[GrampianStars]

G'day Y'all
In the field I use the 17Ah jump start from supercheap [the yellow one]
they've now got a 24Ah kit with multiple outlets on :)

[Karls48]

Little light on lead acid batteries

Any battery testing (to give meaningful result) must be done with a load. Measuring voltage with voltmeter without the load tells you nothing. Common 13.8-volt 7A is rated at 20 hours discharge cycle. That means it will supply 0.35A of current for 20 hours before it is discharged. Increasing the current drawn from battery over the rated discharge current will decrease effective battery capacity. Drawing current smaller then rated discharge current of the battery will increase effective battery capacity. To put it in other words, if you have 7AH battery and you draw 1A from it, it will be able to supply 1A of current for about 5 hours (5 AH). But if, using same battery, you draw only 0.1A of current it will supply that current for about 85 hours (8.5 Ah). For exact numbers you have to look up manufacturer Discharge charts, as it will vary from one brand to the other.
To test 13.8V 7AH battery:
Fully charge battery
Connect 47R 5W wire wound resistor across the terminals
Battery Ampere Hours / Discharge rating (7/20=0.35)<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
Resistance needed to get 0.35A at 13.8V – Voltage * current (13.8*0.35=48.3). Nearest value available is 47R.
Now measure voltage across the battery terminals:
13V – good
12V and above - OK
11V – and above no much left
11V and below – stuffed.
You may rejuvenate (sometimes) stuffed battery by discharging it to 10.5V and then recharging it slowly. Repeat process 5 to 6 times.
Regards
Karls48

[Vermin]

I've never had any luck trying to rejuvenate SLAs, which is why I have a battery voltage monitor with audible low alarm - to prevent the battery discharging below 11V.