# Battery Consumption / Calculation  - Useful info!



## GRWXJR (Feb 7, 2013)

I was just looking about and came across this site.

Lots of useful info about connecting batteries for the lay-person (that'll be me then ) and a handy calculator feature so you can run 'what if' scenarios to work out how long your storage could last with different loads.

I wanted to check against my own figures how long my meagre storage of 1 x 75AH Leisure Battery could last now I've put LED's in to reduce consumption, and its nice and easy to do here:

Notes on Batteries

All the best.  G.


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## Firefox (Feb 7, 2013)

Probably the most important thing to realise is battery condition and charging efficiency are going to have more impact on what capacity you can get than formulas. But there aren't any forumals to work out how well you have charged or how damages/sulphated your batteries are.

Theoretically you can add all your wattages together work out your current draw and apply the peukert thing and work out how long you've got.

Practically you can take your battery capacity say 100 Ah and multiply by 50% = 50 Ah to avoid deep discharge and further damage. Now take that 50 Ah and multiply by 80% to allow for inefficient charging, another 80% to allow for poor condition, and another 80% for rapid discharge effects. 0.8x0.8x0.8 is another 50% Giving you 50Ahx 0.5 = 25 Ah.

Basically any 1 or 2 year old battery, whatever capacity it says on the tin you can divide that by 4. So a 100Ah battery becomes 25Ah or an 80 Ah becomes 20Ah. This is why people who have old leisure batteries or ones they got free from a friend wonder why their batteries last for so little time.


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## GRWXJR (Feb 7, 2013)

I take your points Firefox.  I just thought that it might be a useful tool as a starting point for lots of folks to whom electrickery is a closed book (esp. if wondering how much storage you'd need or if putting in new batteries what to go for).

I do know a little bit about batteries and lecky, but it was still nice to have the calculator support my own estimates.

After all, any of us can add in a de-rate factor for an older battery after using the calculator, or it could help someone work out what they OUGHT to be getting if they are concerned that their battery might not be up to snuff maybe?

I liked it anyway :dance:


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## Firefox (Feb 7, 2013)

I'm not really knocking the calculator, if it seemed like that  It works fine assuming your battery is new and perfectly charged. Just a caveat really for people using it, that very rarely are the batteries they are using in that condition. 



  There's a great tendency generally to think a 100 Ah battery means that's how much you start with.

For the average motorhome  (if there is such a thing!) 80 to 100 Ah is fine for weekenders or campsite users, wilders will probably want 200 Ah and fulltimers+wilders 300Ah. Any more than 300Ah in the average motorhome eats into the payload and space rather too much for most people's comfort.


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## GRWXJR (Feb 7, 2013)

*Battery Oomph!*

@ Firefox  





> There's a great tendency generally to think a 100 Ah battery means that's how much you start with.



Yep - tis a good point that.  Even with a spanking new un I work on 50% (so 50Ah for a 100AH rated battery) as the actual 'useable' capacity so that you don't spank the thing and wreck its longevity.

IF my 75AH battery was fit (& it isn't, but then with what I spent on the van with all it came with I'd be an eternal optimist if I expected all to be perfect), then I estimated max 24 hours use with a draw of up to 1.5A.

75 / 1.5 = 50 Hours, but then divide that by 2 to allow for the 50% rule is how I looked at it - which is pretty consistent with your statement that a (good) 80 - 100AH single 12v should be good enough for a weekender like I expect to use the van for :cool1:

But as my 75AH is basically FUBAR and needs replacing , I know it won't last anything like as long as that (but at least my calcs prove that the thing is kaput without me needing a decent disharge tester or other gadget (there's one in work I have access to, but lots of folks won't have access to a £150 Digital Battery Discharge Tester).


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