NiMh, SLA and other electric bike battery types



22nd November, 2013 in Buyers Guides

Lithium based batteries have become the standard for almost all e-bikes in the western world since they are so suited to this application. However, there are other battery types that you may come across and I’ve included information of a battery type no longer in production for those researching battery types on second hand e-bikes and kits.

SLA batteries

These are sealed lead-acid batteries, similar to those used in cars, but having paste rather than liquid electrolyte so that they can be used in any orientation. Used chiefly on low cost e-bikes since they are much less expensive than lithium types, they have some disadvantages. First they are very much heavier. Second they can have a shorter life in e-bike use. Third, due to something called the Peukert effect, they cannot give up all their charge content in high discharge applications like use with e-bikes, so range can be as little as half the expectation for a given capacity, depending on the motor power.

SLA batteries never need to be conditioned, but otherwise they should be treated the same as lithium, charged as often as possible. They usually take the form of two or three sealed 12 volt units which users can easily replace from within their outer case. When buying replacement 12 volt SLA batteries, it’s important to get the high discharge types and not those designed for long life, low current applications like burglar alarm standby power.

NiMh batteries

Once the main type, Nickel Metal Hydride batteries are still used in a small minority of e-bikes. In weight they fall between lithium and SLA, but much lighter than the latter. Unlike lithium and SLA batteries, their charge content is best used as fully as possible each time, but if that’s not practical, about once every 20 to 30 charges the battery should be emptied as much as possible by riding the bike to that low charge state. This is due to a tendency to suffer some small degree of “memory effect”, this explained in the NiCad section below.

Never discharge an NiMh battery using external means like light bulbs or resistors. Their cells have minimum safe voltages below which they can be destroyed. E-bikes have low voltage cut-offs that protect batteries against this happening.

NiCad batteries. Large cell Nickel Cadmium batteries suitable for e-bikes are no longer in normal production due to the environmental toxicity of cadmium. Some were still in use until recently though, chiefly by Heinzmann motor kits. Although they only have about half the capacity of NiMh for a given cell size, in one way they were ideal for e-bike use since they could freely give up their current at very high discharge rates. However, as well as the toxicity, they had the disadvantage of a “memory effect”. This meant that if part charged by topping up, the battery cells could chemically set up a barrier and thus “remember” the point from which the charge was applied. The effect of this was is that only the amount of the top up charge down to the chemical barrier could be used sometimes. This meant that they were best fully discharged before every charge, which was inconvenient.

Carefully designed charging techniques minimised this effect and cells which were locked in this way over time could be recovered by a special conditioning process applied by main dealers. However, these inconveniences meant NiCad was quickly superceded by NiMh for all but Heinzmann, a few of which are still in use.

SCiB batteries.

This is a lithium-titanate battery type. The name SCiB (Super Charge ion Battery) is Toshiba’s who supply Schwinn, at the time of writing the only maker of e-bikes to have used these. As a lithium type they can be treated in almost all the same way as all other lithium batteries. The major difference is charging, since this is a very fast charging type, the supplied charger typically completing a full charge in 30 minutes. The disadvantage preventing their greater use is that for a given volume and weight they only have half the capacity of most other lithium batteries, so for adequate range are best suited to low-power assist motors.

Our thanks for Flecc for this article.


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