9V Battery project - UK Vintage Radio Repair and Restoration Discussion Forum

valvekits · 11th Feb 2010, 12:49 pm

I have a project where space is limited, that requires a rechargeable battery that can supply 9V @ 290mA for several hours. I’ve considered two rechargeable possibilities: -
1. Eight 1.2V AA 2800mA batteries in series
2. Ten PP3’s in parallel
Route 1 seems to have an obvious pitfall in trying to charge the cells whilst in series.
Route 2 will give me less mA/Hour but looks more feasible for charging.

Would anybody care to share his or her thoughts with me on this?

Cheers
Eddie

AlanBeckett · 11th Feb 2010, 1:07 pm

Eddie,
Why should charging 8 cells in series pose a problem? That's how cell stacks are usually charged. Assuming they are NiCds or NmH then 0.1C (280 mA) for 14 hours from your trusty bench PSU will sort them. Just make sure they're all new to start with.
Alan

ppppenguin · 11th Feb 2010, 1:54 pm

Charging NiCd or NiMH in parallel is often a bad idea. The current sharing is pretty indeterminate. Series charging is absolutely fine.

valvekits · 11th Feb 2010, 4:45 pm

Ah okay, in my limited experience with rechargeable batteries in the past I had found that they discharged to different values between 0.6V and 0.8V. I reasoned that if I charge them (series connected) and I determine that the end of charge period is 9.6V, then some cells are going to get overcharged whilst some cells would be undercharged. Could it be that my logic is flawed and regardless of what they discharge to, they can only charge back to 1.2V?

Cheers
Eddie

ppppenguin · 11th Feb 2010, 4:55 pm

Very flawed logic. Charging NICd and NiMH is basically a constant current job. Unlike lead acid. You can charge NiCd for long periods at C/10 (100mA for a 1Ah cell) without harm. At higher currents you need intelligent chargers to prevent overcharge. Just measuring terminal voltage is not enough.

If the cells start in very different states of charge then some will be fully charged before others. This can cause harm at high charge rates. Ideally all the cells in a battery should always be used together. A long slow (C/10 or less) charge should equalise them when connected in series.

Brased · 11th Feb 2010, 5:25 pm

A rechargeable PP3 consists normally of six cells connected in series (which provides rather less than 9V) so in practice this construction also relies on the individual cells being reasonably balanced. This should also be the case if you buy a batch of single AA cells.

The best solution would be to use AA cells and a proper modern charger which controls the charging of each cell individually (normally four at a time although some will take more) but I appreciate that this is a relatively expensive solution.

valvekits · 12th Feb 2010, 11:06 am

Okay I'm going to have a play with series connected cells and see how we get on. Thanks for all your input.
Mods I think we can close this one.

Cheers
Eddie

11th Feb 2010, 12:49 pm	#1
valvekits Pentode Join Date: Dec 2008 Location: Nottinghamshire, UK. Posts: 239	9V Battery project I have a project where space is limited, that requires a rechargeable battery that can supply 9V @ 290mA for several hours. I’ve considered two rechargeable possibilities: - 1. Eight 1.2V AA 2800mA batteries in series 2. Ten PP3’s in parallel Route 1 seems to have an obvious pitfall in trying to charge the cells whilst in series. Route 2 will give me less mA/Hour but looks more feasible for charging. Would anybody care to share his or her thoughts with me on this? Cheers Eddie

11th Feb 2010, 1:07 pm	#2
AlanBeckett Rest in Peace Join Date: Dec 2008 Location: Burton upon Trent, East Staffordshire, UK. Posts: 1,686	Re: 9V Battery project Eddie, Why should charging 8 cells in series pose a problem? That's how cell stacks are usually charged. Assuming they are NiCds or NmH then 0.1C (280 mA) for 14 hours from your trusty bench PSU will sort them. Just make sure they're all new to start with. Alan

11th Feb 2010, 1:54 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: 9V Battery project Charging NiCd or NiMH in parallel is often a bad idea. The current sharing is pretty indeterminate. Series charging is absolutely fine.

11th Feb 2010, 4:45 pm	#4
valvekits Pentode Join Date: Dec 2008 Location: Nottinghamshire, UK. Posts: 239	Re: 9V Battery project Ah okay, in my limited experience with rechargeable batteries in the past I had found that they discharged to different values between 0.6V and 0.8V. I reasoned that if I charge them (series connected) and I determine that the end of charge period is 9.6V, then some cells are going to get overcharged whilst some cells would be undercharged. Could it be that my logic is flawed and regardless of what they discharge to, they can only charge back to 1.2V? Cheers Eddie

11th Feb 2010, 4:55 pm	#5
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: 9V Battery project Very flawed logic. Charging NICd and NiMH is basically a constant current job. Unlike lead acid. You can charge NiCd for long periods at C/10 (100mA for a 1Ah cell) without harm. At higher currents you need intelligent chargers to prevent overcharge. Just measuring terminal voltage is not enough. If the cells start in very different states of charge then some will be fully charged before others. This can cause harm at high charge rates. Ideally all the cells in a battery should always be used together. A long slow (C/10 or less) charge should equalise them when connected in series.

11th Feb 2010, 5:25 pm	#6
Brased Hexode Join Date: Mar 2005 Location: London, UK. Posts: 318	Re: 9V Battery project A rechargeable PP3 consists normally of six cells connected in series (which provides rather less than 9V) so in practice this construction also relies on the individual cells being reasonably balanced. This should also be the case if you buy a batch of single AA cells. The best solution would be to use AA cells and a proper modern charger which controls the charging of each cell individually (normally four at a time although some will take more) but I appreciate that this is a relatively expensive solution.

12th Feb 2010, 11:06 am	#7
valvekits Pentode Join Date: Dec 2008 Location: Nottinghamshire, UK. Posts: 239	Re: 9V Battery project Okay I'm going to have a play with series connected cells and see how we get on. Thanks for all your input. Mods I think we can close this one. Cheers Eddie