Charging a PCB battery?

[Doomguy42]

Hi, I'm putting a board into storage. The Pcb cmos style battery has been flywired away from the battery in case it leaks.

The battery is a Varta 2.4v (see picture)

I'd like to be able to keep the battery charged while the board is not being used to retain the memory and also to stop the battery leaking.

Can I use a simple power supply to charge it up?
I was thinking 3v but I'm not sure on strength of current and over charging the battery?

I'm not sure if there's any more factors to consider? Can anyone help?

 

[Nigel Goodwin]

3v should be fine, as would 5V (or anything else really), but you MUST use a current limiting resistor - you're only wanting a small trickle charge to keep the battery topped up. You should be aware that this won't stop it leaking, these types of batteries leak because they are old, not because they are flat.

However, are you aware that those batteries are pretty unreliable, and only have a finite life even if left on trickle charge. You should change them periodically (before they die) as part of your servicing procedure - this is why burglar alarm companies replace the batteries after a certain number of years.

 

[Doomguy42]

Yes thanks they are troublesome but should last a few years if they're kept in ok conditions. I have repaired that board from previous leak in the upper center above the RAM. I keep an eye on them an replace them often.
That's great info. I'm thinking I do this next-
1. I measure the charging current the battery gets with the board is running.
2. Select power supply.
3. Use some type of calculation to work out what resistance I need to reduce the current to a trickle.

I'll post back when I have measurements thanks for the teachings

 

[Nigel Goodwin]

Yes thanks they are troublesome but should last a few years if they're kept in ok conditions. I have repaired that board from previous leak in the upper center above the RAM. I keep an eye on them an replace them often.
That's great info. I'm thinking I do this next-
1. I measure the charging current the battery gets with the board is running.
2. Select power supply.
3. Use some type of calculation to work out what resistance I need to reduce the current to a trickle.

I'll post back when I have measurements thanks for the teachings

You might also try measuring how much current is taken from the battery when it's feeding the board, and then calculate your resistor to provide that much current plus the required trickle charge for the battery.

 

[gophert]

The only NiMH batteries I've seen leak is when they are heated from rapid discharge or rapid discharge - or heated externally. They are very different from Alkaline batteries.

 

[Doomguy42]

I think if they're left flat and in cold humid conditions they seem to leak. In my hobby virtually every board I have has had leaked battery at some point. They were also placed strategically( next to the cpu or Ram) to 'kill' the boards at some point. I still use them because they're perfect and as per the original spec. I just fly wire them and keep an eye on them.

 

[Nigel Goodwin]

The only NiMH batteries I've seen leak is when they are heated from rapid discharge or rapid discharge - or heated externally. They are very different from Alkaline batteries.

'Leak' isn't really the correct term - perhaps 'corrode'? - but it's very common, and very often severely damages the PCB and components.

The OP had the right idea moving it off the board.