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Author Topic: Testing a Sealed Lead Acid battery  (Read 5738 times)

kiwimodeller

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Testing a Sealed Lead Acid battery
« on: March 19, 2011, 10:18:46 am »

Greetings, can anybody advise on how I tell if my 12volt 7 Ah SLA battery is coming to the end of its useful life? I charge it slowly using a small charger made for this type of battery by a local electronics chain. Eventually (usually 12 to 15 hours charge) the voltage gets up close to 14volts however recently I feel that both the speed of the boat and the time it will run for at reasonable performance have both decreased notably. The boat used to look like it was planing and had a great bow wave and I could get this sort of performance for a couple of hours but lately the performance does not look as good and seems to drop off even more with half an hour so I suspect that the battery is becoming sulphated but do not know how to test it at the end of a run the voltage will be about 11.7 volts but will recover to 12 if left for a while. Any suggestions welcome. Thanks, Ian.
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Netleyned

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Re: Testing a Sealed Lead Acid battery
« Reply #1 on: March 19, 2011, 10:56:14 am »

To get it professionally tested will tell you but new batteries are not that expensive
If you put something like a 24Watt car bulb across the battery after a full charge and
the battery is ok you should get about three hours plus at full brightness
If the bulb dims before 2 hours the a new battery is on the cards

Ned
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Le Caux Deux

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Re: Testing a Sealed Lead Acid battery
« Reply #2 on: March 19, 2011, 11:52:09 am »

Great site here
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
and
http://www.batterystuff.com/tutorial_battery.html#6

Lots of other stuff on the net, I find that if a battery gets down bellow about 12.5v its efficacy drops off very quickly. I read somewhere on the net that a fully charged PB battery should show about 13v and that the percentage discharge is indicated by the battery voltage so by 12.5v the battery is down to 60% of its capacity or 0.12v for a 10% drop in capacity. So by less than 12v the battery is fully discharged, and my understanding of PB (LA) batteries is that one should not fully discharge them on a regular basis

Mike

PS
it might be worth checking that the motors are still only drawing the expected ampage and props are not binding or motor getting hotter than expected
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Colin Bishop

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Re: Testing a Sealed Lead Acid battery
« Reply #3 on: March 19, 2011, 12:33:54 pm »

Lead acid batteries should not be discharged below 50% of their nominal capacity otherwise permanent damage is likely to result. This is unlike NiMH and LiPos which can tolerate discharging to low capacity levels.

Colin
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ministeve

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Re: Testing a Sealed Lead Acid battery
« Reply #4 on: March 19, 2011, 08:19:51 pm »

do you live near a house alarm fitting outfit if yes they should have a Pb battery tester. it only takes 1 minute to test and shouldn't cost anything or if you have deep pockets a.c.t meters sell an intelligent battery tester which our lads use at work i have seen them on ebay too they are good gives you the ampage that the batt can give out

Steve
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johno 52-11

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Re: Testing a Sealed Lead Acid battery
« Reply #5 on: March 19, 2011, 08:23:32 pm »

Hi Ian

What boat are you using this in and what's the motor setup.

A SLA battery will only give you its stated amp hours if it is discharged at a rate of 0.05C so to get 7 amp hours out of your battery it will need to be discharged at 0.35 amps

If you discharge it at a higher rate you will get less amp hours form it At 1C 7amps it will only work for about 35mins and give you just over half its stated capacity. If you discharge it at 2C 14amps if will last about 12mins and give you less than half of its stated Capacity. You can find all the facts here http://www.yuasabatteries.com/industrial.php just look for the data sheet on your size of battery at the bottom of the page.

To test what your battery is still capable of holding then you need to find someone with a charger that will discharge as well as charge and that has a display to tell you what it has taken out of the battery as well as what it has put in.

here are a few examples
http://www.component-shop.co.uk/html/body_radio_control1.html look at the two Turnigy half way down.
http://www.graupner.de/en/products/843d18cf-1eaa-4aa4-a3d0-fb05359d6f7f/6468/product.aspx I have the Ultramat 15 and when I charge the 7Amp 12 Volt SLA's  I use in my Arun they only take between 3.5 to 4 amps to be fully recharged and I know that at full throttle the Arun's motors are pulling 12 amps but its not run continuously at full throttle when its on the water so I can get 25 mins running time out of a battery.

If your battery is on its way out there are a couple of options. The chepest is to buy another SLA if the old one worked well in your boat and the running time was long enough for your needs. If reducing the weight would be an advantage then you could look at a 5000mAh sub C NiMH pack. This would be less than a third the weight of your SLA and give you the same running time. The disadvantage is that it would cost more and you would also need a new Charger.
If you want a longer running time then you could look for a NiMH pack of 9000 mAh D cells or 13000mAh f Cells both of these would still weigh less than a 7A 12V SLA disadvantage is the cost and the need for a new charger. I have a F cell pack from my boat and it gives me more than 3 times the running time as the old SLA and of course there is also the LiPo option.
 

 
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