This is a pretty technical page, with no pretty pictures, sorry! In the NiCad packs, Heinzmann use only Sanyo Cadnica Nickel-cadmium cells - 20 F-size cells in the 24V pack, 30 D-size cells in the 36V pack. The F cells are rated at 7 Ah for a 24V pack capacity of 168Wh. The F-cells are rated at 4.4Ah for a 36V pack capacity of 158Wh (note - the 4.4Ah is pessimistic - 5Ah is more realistic, but Heinzmann like to play it cautious!). Pack Life: This is a very difficult thing to define exactly - the capacity of all battery packs declines with age, for a number of reasons. Some of the reasons are reversible (see the “Memory Effect” below), some are not. The general definition is that a pack is “dead” when it has 75% or less of it’s new capacity - by this definition, we’ve found that Heinzmann packs generally last over 1000 cycles - that’s three years of daily use!
The non-reversable decline is generally down to the quality of the cell - the Sanyo cells are generally acknowledged to be the best made (although Panasonics are close...), so last longer than cheaper brands.
Funnily enough, NiCads seem to like abuse - the more they’re used the better they get. The ones which are used daily last very well - the ones which sit unused for 6 months generally need reconditioning sooner! “Memory Effect”: This is often talked about with reference to NiCad cells - the idea that NiCad cells must be discharged fully each time to get the full capacity next time. There is no such thing as the “memory effect”. There are several effects which do contribute to the temporary loss of capacity:
Elevated temperature charging/voltage depression: This is a common problem with less sophisticated chargers, and the effect is to make it appear as if the cell only “remembers” the last charge it got. A good charger which measures dT/dt (like the Heinzmann one) won’t charge the pack incorrectly like this.
Crystallisation on the cell plates: NiCad cell plates are very thin and rolled up - the idea is to get the maximum surface area possible. With age, the active compounds on the plates can grow into larger crystals than normal - they then have a smaller surface area, so capacity is reduced. This generally takes at least 18 months of daily use to become noticeable, and it can be reversed by reconditioning the pack. Reconditioning: The above effects can be reduced or eliminated by reconditioning the pack - we have the sophisticated charging and discharging equipment to do this. In practice, we’ve found that we can get a 3-year-old pack which has had well over 1500 uses back to full capacity, although of course we cannot guarantee this. Reconditioning on our system costs £50. Cell replacement: Eventually, all batteries do fail. We can replace your NiCads, keeping the rest of the Power Pack intact - cheaper than buying a new one! New cells are £225. I know this sounds expensive compared to our competitors, but look at it this way:
Heinzmann: £225 every 1000 cycles = 22.5p per cycle (maximum! We can probably recondition your pack to get a lot more life for your money)
A well-known competitor: £70 every 250 cycles = 28p per cycle (if we’re being optimistic about how long the lead-acids last!) What we do with the old cells: Cadmium is nasty stuff, so we do not just dump the old cells in a landfill site. The old packs are given to a recycling company who crush them, separate out the chemicals for reuse, and recycle the steel containers. This is not cheap, as the chemicals aren’t worth much, so £25 of the cost of replacing the cells goes towards the recycling. A price well worth paying I hope you’ll agree! Other cell types: Now available are NiMH cells in the silver box configuration - they give you 60% more range for the same size and weight! Heinzmann spent two years working with Sanyo to make sure the cells were reliable enough - and also developed new chargers and controllers to protect the slightly more delicate NiMH cells.
Heinzmann are currently looking at Lithium Ion and polymer cells, but are not yet happy with the lifespan or reliability.
Lead-Acid batteries just aren’t worth the bother - yes, they’re very cheap, but you can’t fast-charge them, they’re heavy, they’re delicate, and they only last 200 cycles. No thanks... The textbook: If you want to learn a lot more about batteries and battery technology in general, then the book to have is “Rechargeable Batteries Applications Handbook” from Gates (ISBN 0-7506-7006-1). It’s sometimes available from Amazon for £45, but you can get it from RS Components for £25. If you want a copy but don’t have an account with RS, give me a call and I’ll get you a copy. | 
