Big NiCads

[DavidTocher]

I've got a battery of 10 unsealed NiCad cells which were aquired from a standby power supply. Each cell is rated 1200 Ah. From the specs they seem to have the current supply capacity to start an engine but I haven't tried it.

I was thinking about using them in a boat but the standard alternator controllers are designed for lead-acid cells. The charging regime required for NiCads is quite different.

Any suggestions; is the idea practical, does anyone supply a suitable alternator charger?

 

[WayneS]

The problem with NiCad's is that they have a memory.

To use them efficiently you must cycle them from fully charged to fully discharged before charging them. If you do not do this then they lose efficiency.

Basically, if you repeatedly take them from 100% to 80% (Something that starting and engine may cause) the recharge them, their fully "discharged state" will become what used to be your 80%. whew... what that means is that their useful charge would be greatly reduced.

In the same fashion, if you only charge them to 90% before starting the discharge cycle, you will soon only be able to charge them to 90%.

Apart from this, they are very good for delivering high currents over a short period. Btw, do not short them or they will blow up.

My recommendation, do not bother, stay will Lead Acid.

Cheers

Wayne

 

[rogerm]

It depends on how many cells are in your battery pack. The more usual 10 cell gives you just 12v dead (ie 1.2v cell). If you have an 11 cell pack then that gives you 13.2v which will usually work very well at pretending to be a lead acid battery. Unsealed ni-cd cells are practically indestructible but for a given volume are only about 1/2 the capacity of a lead acid battery.

So I would say if you have an 11 cell battery (or can add a cell to a 10 cell battery) then go ahead and use it. If you are stuck with 10 cells then if you try and use it you will find the electrolyte keeps boiling off ie they are being overcharged.

One thing to remember with unsealed ni-cd is that they have the memory effect like every ni-cd. To get back the capacity (and DON't do this with any other type of battery...) once a year run the batteries absolutly flat, down to 0v using a load such as a car headlamp bulb (or bigger if you can find it). When down to 0v (and not before!) actually put a short across the terminals and leave it for 24 hours before charging them up again. This is especially good at equalising the cells.
Roger

 

[peterb]

Electrolyte

And if the electrolyte level falls don't try topping up with battery acid! The electrolyte used in NiCads is alkaline.

 

[peterager]

I seem to remember that it is usually only possible to recharge nicads a fixed number of times.
Don't quote me but I think it's about 500 times. This is not as good as it sounds, because it depends upon the charge rate and terminal voltage when the charging starts.
Also do not overcharge, they tend to get hot.

I agree with Wayne, lead acid is a much safer bet.
Peter

 

[halcyon]

Robin Davey used them in Cornwall the Whitbread single hander around 1994. got round the world without a problem. These were sealed though, and he had a TWC because he needed 15.5 volt to charge them, though this is memory.
I would have serious worries though about using unsealed and secondhand ones, by the time you have sorted out charging and installation, it will be cheaper to buy normal lead acid.



Brian