Battery substitution

[greg_simons]

Just started work on a Sinclair mtv1 mini tv, among other things the original nicad batteries are dust, literally, after over forty years, knowing the problems with this type of battery and the difficulty replacing them are there any other types of cell chemistry that could be used, need an expert opinion.
Greg.

[agardiner]

These sets were designed to take 4 standard AA batteries, and they did come with a pack of 4 Ni-Cds. So you can happily just use 4 modern Ni-MH AA cells in their place, and should be able to just drop them in and use a regular charger.

[broadgage]

And the modern cells will be of greater capacity. Back in the day, AA Nicads were often 0.5 AH, these days 2.0 AH are common.

[greg_simons]

Thanks both, sounds like a plan, was just slightly concerned about any disparity in charge current, although Sinclair rather loftily state that the circuitry is optimised for maximum battery life, which probably means they weren't ramming the maximum charge current through them, I'll look into a nice set of nicke metal hydride cells, once I've cleaned the remaining guts of the originals away!.
Greg.

[Bazz4CQJ]

Isn't it the case that a charger designed for Nicads won't get the best life out of NiMH?

Isn't it the case that a charger designed for Nicads won't get the best life out of NiMH?

Yes, but the MTV1 was pretty basic on the changing front just a resistor from the plug in PSU. That is OK for NiCd, they can take 1/10th of capacity for ever. NiMH don't like much more than 100th after being full. Having said that just charge for a day or so, all will be fine.

Do take care replacing the cells, I blew mine up making a very silly mistake (I dropped the solder across the battery and one of the chips, oops, nice ornament!).

[agardiner]

I wouldn't worry about the charging; given the uplift in capacity, it will be trickle charging the new cells anyway.

[broadgage]

Are the cells fixed in place, or are they removable? If fixed in place I would tolerate the "incorrect" charging circuit, if the original cells were 0.5 AH then the charging current was probably about 50 ma which would fully charge the cells in about 14 hours, and could be continued for a day or two.
This current will charge modern 2 AH cells in about 50 hours or two days and a bit. PROLONGED overcharging will eventually damage modern cells, but it will take a long time at such a low current. 2.5 AH cells will take longer in rough proportion, and also take even longer to suffer damage from overcharge. Replacements are cheap.

If the cells are readily removeable, then charge them in an external charger.

[Craig Sawyers]

Most modern smart chargers detect whether it is niMH or NiCd and adjust the charging cycle to suit. And charge end point is different for both cell chemistries, so it is important to have the correct charging system.

Shoving NiMH cells into a Sinclair product intended for NiCd will almost certainly cook the cells in due course.

Craig

[greg_simons]

Thanks broadgage, yes the cells are soldered in and the whole tv has to be stripped down, indeed the service data suggests ,14 hours to a full charge so doesn't look like any particular damage will be caused with such a low charging current.
Greg.

[ajgriff]

You can still buy AA NiCad batteries so I'd be tempted to replace like with like.

Alan

[greg_simons]

Hi Allen, yes, I'd contemplated that, tbh i was trying hard to avoid the dreaded memory effect by using a more robust cell type in view of the work involved to replace them, plus they're not terribly plentiful, and expensive.
Greg.