Replacing 250mAh NiMH with higher capacity

[Grumps]

Hi All So, as per subject. This is for some solar lights that have 250mAh NiMH

1.2V AA cells, but would 2500mAh cells work too?

[The Natural Philosopher]

if they are the same chemistry and fit physically yes, with the proviso that some very cheap chargers 'expect' a certain cell capacity

[Chris Hogg]

Assuming they are the same physical size as the 250mAh cells and fit, then in principle they should give you light for longer, appropriate for early winter evenings and long nights. But the converse is that they'll take longer to recharge, inappropriate for short dull winter days.

[tabbypurr]

sure no problem. They'd last longer too as they only see part of a cycle each day. If the things contain more than 1 cell, do your best to capacity match them or they'll see repeated reverse charging which kills them much quicker.

NT

[Theo]

Beware that higher capacity cells can have lower cycle life (number of charge/discharge cycles before they die). Given solar lights only get a certain amount of energy per day you might find the extra capacity isn't much use, and yet they die faster due to the lower cycle life.

For comparison, Eneloop Pro have 500 cycles life, Eneloop Lite have 3000 cycles life. So the Pro would last 1.5 years and the Lite 8 years.

Theo

[John_j]

Yes.

But the converse is that

No, you should get the same amount of charge added and so get the same result even on short dull winter days.

Very crude simple chargers may limit the amount of charge the cell gets to avoid overcharging so while it will still work with the higher capacity calls, you won't see any advantage when using higher capacity cells.

[tabbypurr]

You ignored the fact that the bigger cells won't get fully charged.

If the 0.25Ah cells are 3k cycle & the 2.5Ah are 500 cycle:

0.25Ah: 3k cycles of full charge = 3k [summer] days of life expectancy. More days IRL because winter only part charges them.

2.5Ah: 500 cycles of 1/10th charge gives 5k days of life expectancy.

These assume no reverse charging due to unequal capacity + total discharge. Hopefully the controller stops discharging before they're fully flat.

NT

[Roger Hayter]

This is complicated by the auggestion that the number of cycles may be geater it the cell is only partially discharged each time.

[Theo]

That's true, but it really depends on that cutoff. NiMH cells really don't like being run flat. If the charging is 40% to 70% then you can get many cycles on them, but if the charging is 0% to 30% then it isn't very good to them.

Also garden lights have a habit of being alternately frozen and cooked, which isn't ideal for the cycle life.

Theo