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[ray t]

I am using two 12v 7ah SLAs in my boat in parallel.  I am thinking of getting a digital volt meter like this one http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=150486934371&ssPageName=STRK:MEWAX:IT but one that measures lower if I can find one.  My question is, at what voltage, when the pumps are not running should I stop using the boat and change the batteries over?

Secondly, anyone recommend a good voltage meter that will work the range I need?

[mattycoops43]

Personally I would rather get a multi charger. This will tell you the start and end voltage of a battery while charging, and will tell you exactly how many mAh go into it when charged. I find this a more reliable method. if you have a 7Ah cell, and it will only take 3000mAh it will tell you that.

They do not have to be expensive, I got a turnigy 6 lipo cell charger that charges nicad, nihm, Pb, and lipo. 6 s lipo is a horrific voltage so it will cope with any number of ninm you care to mention and it was somewhere around the £35 mark. Most model shops sell them.

I realise you asked specifically about a volt meter, but having bought one of these, I would never go back!

Matt

[Shipmate60]

ray t,
The best way to know when the battery needs changing is with the pump running.
SLA batteries will show a higher reading when not under load, but as soon as a load is applied the voltage drops.
This will depend on the current draw of the pump(s).
A heavy current draw will naturally drop the voltage slightly when fully charged, a low current draw shouldn't unless the battery capacity is suspect or the battery is discharged.

Bob

[ray t]

Thanks for that Bob, makes sense I guess.  So would you suggest running the two aft pumps in the edge of the lake and then checking the voltage? Might prove a little difficult though as the boat will be trying to get away.  Unless, that is, it is ok to run bolge pumps without any water for a short time!!!

[ray t]

Personally I would rather get a multi charger. This will tell you the start and end voltage of a battery while charging, and will tell you exactly how many mAh go into it when charged. I find this a more reliable method. if you have a 7Ah cell, and it will only take 3000mAh it will tell you that.

They do not have to be expensive, I got a turnigy 6 lipo cell charger that charges nicad, nihm, Pb, and lipo. 6 s lipo is a horrific voltage so it will cope with any number of ninm you care to mention and it was somewhere around the £35 mark. Most model shops sell them.

I realise you asked specifically about a volt meter, but having bought one of these, I would never go back!

Matt

Sorry, I think you may have misunderstood me.  I need to be able to tell when the batteries need swapping out when I am bankside.  I know I could wait until the boat just stops, that would prove a pain if the boat was still 150yrds out across the lake 

[Shipmate60]

ray,
If you can take the running amperage of the pumps then divide this into 7 Ah then only use about 75% you should have a factor of safety.
Or just wait till the performance drops off then bring her in as SLA batteries don't dump like MiNim or NiCads so give you more warning.

Bob

[mattycoops43]

Sorry, I think you may have misunderstood me.  I need to be able to tell when the batteries need swapping out when I am bankside.  I know I could wait until the boat just stops, that would prove a pain if the boat was still 150yrds out across the lake 

LOL Yes, totally! Don't worry. I thought you meant your batteries were coming to the end of their life and you wanted to know a good way to judge their performance.
 

[tigertiger]

I would have thought the boat would slow down a long time before the batts die.
You could check this by wacking the boat up to full speed for a second of two, even if not to scale speed, to see how much life is in the batts.

But what do I know.

[ray t]

OK guys, cheers for that.  Sounds like installing a battery monitor may be a waste of time then?

[Tugwash]

its a bit pointless when the boats 150 yards out cos you can't see the meter....

But it could be useful to tell at bank side, a trick i have used in the past to test running time is basically tether the boat to the bank and set the stop watch going and run the models battery's until they die, this will give you a good idea of the running time , then when your out on the water next time untethered you just keep watch on the time bring it in 15 mins earlier this way you have enough reserve in the battery to make it back to shore with no worries about trying to retrieve a stranded model