How to pick a lead battery...

[Burgerman]

This is the easy way.

Reserve capacity is a measurement of how long a battery can sustain a 25A load. In minutes. So it is an easy direct measure of performance under load.

An MK GEL 74Ah reserve capacity is exactly 135 minutes. Its in its spec sheet. The clock stops when the battery voltage falls to 10.5V (21V in a chair).
when discharging at exactly 25A. Unfortunately for us a chair stops and gives a red flashing at some point around 22.5 to 23V. So we dont get to use it all!

So in reality you dont get that full 135 minutes. Maybe 90 to 100 mins at 25A. It doesent matter a lot it still allows DIRECT battery to battery comparison under load.

So other batteries with worse internal resistance fall to this point faster. You dont go for as long... It isnt about Ah like a fuel tank with lead, but about how many Ah are ACCESSIBLE at any given load. That 25A figure equates to a heavy rehab 6.2 or 8 mph chair rolling at full speed quite well. We get around 14 to 15 miles before the battery dies, in general which is 2 hours (120 mins) at full speed on most heavy chairs.

So when looking for a decent chair battery its an easy way to compare performance when you cant find or understand internal resistance values or the Ah degradation due to load etc. Now this tells us nothing about its cycle life. A starter battery usually has great reserve capacity and is cheap. But wont last a month.

Just choose the battery with GOOD cycle life TOO!
(say 450 to 500 at 80% discharge levels) and the best RESERVE capacity rating like 135 minutes, or 120 mins etc in order to find the one with the best performance/range and lifespan.

Then look at the price.

[Burgerman]

The chair stops before that battery reaches the reserve capacity figures 10.5V (21V) point.

You get about 2 more blocks when you see this!

[Burgerman]

Also it gives an easy way to compare to lithium.

MK gives 135mins at 25A. Actually about 100mins because we dont function at 21V... We will ignore that.

So I MEASURED 244Ah from the EVE Lithium 230Ah cells down to 24V. (2.7 X 8 cells). Thats 21.6V.

Lets still call it 135 mins for lead, and punish the lithium. It wont care. Because lithium suffers zero peukert capacity at a "tiny" 25A load, and gives the full 244Ah regardless. So how long will that run at 25A? Well thats easy, 244 div by 25A! Its exactly 9.76 hours. Thats 9 and three quarter hours. Or 585 minutes. Or 4.334x longer.

Except its not quite right because instead of the chair stopping at some point near 22V the lithium keeps going a bit longer as its voltage is a bit higher even here - it can get more discharged before the chair dies on you... So will add extra range. But we will ignore that here.

IMPORTANTLY because the voltage is a fair bit higher all day long - even when full - we have around 10% or so lower battery drain at any given load/speed - all day. So thats 585 mins + 10% (or more) so add another 58.5 minutes run time... So now its 643 mins. And because the battery is half the weight, add some more! (we wont bother) So now it 643 minutes or 10.72 hours... Divided by 135mins. Thats 4.83 times the range ignoring the big difference in weight too.

And this is about what the other way of working out the range difference also gives us. And seems accurate in practice. The stored watt hours way to calculate this at the 2 hour rate says around 5.25x the range of MK.