Originally Posted by Hamwich
Knowing that there are loads of clever types out there who know much more about this kind of thing than I, here's a question:

I am thinking of taking my Paddleboard somewhere where the Landy can't get to (more correctly, isn't allowed) so I'm thinking whether it's worth buying a small 12V battery to carry to power the electric pump. The pump is 110W and can inflate my board to 18psi in about 10 minutes.

Rounding out to 120W for 15 minutes suggests I need 2.5 AH. Allowing for inefficiencies, and possibly the need to inflate a couple of times, it would seem that a little 7 AH deep cycle battery like this would be more than adequate capacity wise:

[Linked Image]

But the question in my mind is: Would the battery be happy delivering 10 amps for 15 minutes without getting all hot and bothered? Part of me is theorising that they are used for things like rechargeable strimmers or mobility scooters so they should be ok, the other part of me is concerned that such a small light battery really might not be up to the job of delivering that much current even just for 10 minutes.

Of course, the alternative is to take the manual pump, but anyone who has manually pumped a paddle board up with one will understand why I'm asking the question smile

A Mr Peukert discovered in 1897 that the effective amp hour capacity of a lead/acid battery reduces noticeably with increased current drain. For this reason larger deep cycle batteries often have one or more capacities quoted according to a constant discharge current with the lower rate giving the highest capacity. A rest period after a short high current drain can allow the battery to recover to some degree but a constant high discharge current may leave the battery with only a fraction of its rated capacity.

An ideal capacity for a 10 amp constant drain would be close to 100 amp hours (C/10) if its not to impinge on battery longevity. A trade off against same would be a discharge rate of up to C/3, requiring a 30 amp hour battery. Similar rates apply to charge currents as well.

It's all to do with the bottleneck created by electrolyte to plate surface area and the rate the chemical energy change can be absorbed and released from the depth of the plates.

Richard

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