Battery recharge circuit - stop when full?

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Fordy

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Ok, here's what I have at the moment:


Vcc = 12v rail.
The SPST switch is marked as for test purpose, as it is to prevent the DPDT relay from remaining in the same position during simulation (Software doesn't allow me to control Vcc on the fly).

The relay is to switch the PP3 battery from being charged, to supplying the main circuit with the good stuff.

My problem is that I don't know how to stop the battery being charged when it has reached its capacity.

I thought of doing a timed charge (which wouldn't be a problem) through a 555 or something, and then a reed relay. But this is supposed to fit into quite a small box... I already have two relays, three incl one DPDT might be a bit of an ambitious PCB.

Thanks,
Fordy.
 

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If you want to prevent the PP3 battery from being charged then where is the battery that you want to be charged?
The 2N2222A transistor is connected upside-down.
 
It is the PP3 I want charged. I said "prevent being charged when reached its capacity".

I also thought transistor was wrong, but it works fine in the sim, and flipping it only slightly lowers voltage and current.

When 2N222 flipped, PP3+ reads 9.73v, as I understand it slightly more than rated voltage is required to recharge. Is that correct? And is .73 enough extra?

Also, as per original question, am I correct that I need to stop it from "overcharging" because of damaging it, and overheating etc. - if so, how do I do this?


Thanks for the quick response,

Fordy.
 
You forgot to say if the PP3 is a 7.2V Ni-Cad, a 7.2V Ni-MH, an 8.4V Ni-Cad or an 8.4V Ni-MH.
You should use a battery charger IC that is set to the chemistry and number of cells of your battery.

A 7.2V battery has six cells and is about 8.4V when fully charged at a fairly high current. An 8.4V has seven cells and is about 9.8V when fully charged at a fairly high current.

I have an 8.4V PP3 Ni-MH battery that is simply trickle charged continuously from 9.0V with a 390 ohm current-limiting resistor for years. It gets close to a full charge.

A transistor with a normal current gain of 200 has a current gain of about 5 when it is upside-down. The transistor in your circuit does not need much gain.
 
I hadn't got as far as parts yet...

So, if I were to use a 8.4v NiMH PP3, my circuit, with transistor rotated from original attachment, that would be fine?

Or would you advise getting it down from 9.73 to a clean (or round about) 9.0v?
 
The charging must be stopped when the battery charges to about 9.4V so 9.73V is too high.
Your circuit has nothing to limit the current to only18mA so the battery will probably blow up anyway.

Your circuit is horrible. Its output voltage changes when the temperature changes.
 
Ok, I had no idea what I was doing really.

Can you post a picture of yours? Or tell me how to change mine to work properly?

Thanks.
 
This is my extremely simple PP3 battery charger that does not fully charge the battery but it trickle charges the battery safely for years.
 

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Ah. Ok, I see.

So if the 9v source goes off, battery has charge, waits a while, gets used and n the process (for my project) turns 9v source back on, which it then gets topped up from and then 9v goes off again, process repeats - it's really that simple?

And apologies for stupid question, but resistor would be connected to +9v and PP3+? Would PP3- go directly to 0v/gnd source or is it not required at all for recharge?
 
If you do not connect together all the grounds to the 0V (the negative terminal of the PP3 battery) then nothing will work.
 
Thanks.

So provided the battery shown here is a 8.43v PP3, should work fine?

Last question, I'm not having much luck finding 8.43v models, all listed as 9v. Is it possible to work out exact pd from the mAh?

I found a 200mAh model here:
**broken link removed**

And slightly cheaper 170mAh:
**broken link removed**


But are either of those suitable?


Thanks for all help.
 

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Fordy said:
So provided the battery shown here is a 8.43v PP3, should work fine?
Click to expand...
I have never bought a battery online without any details.
I have never bought a no-name-brand battery.
I have never bought a battery in the UK.
I buy American Name-Brand batteries on-sale at a local store in Canada after reading the detailed datasheet and label.

The first PP3 battery might be a modern low-self discharge one but the ad does not say how many cells it has so we don't know if it is 7.2V (6 cells) or 8.4V (7 cells). Since its capacity is pretty high at 200mAh then it might have 6 cells (7.2V).
The second PP3 battery seems like the ad was started 3 years ago so maybe the battery is the old type that self-discharges quickly. Its capacity is lower so maybe it has 7 cells (8.4V).

My circuit charges my 8.4V battery with about 1.5mA from only 9V so it is never over-charged.
Your circuit charges a 7.2V battery with about 12.3mA from 12V and severely over-charges it. It charges an 8.4V battery with about 9.2mA and also severely over-charges it because the 12V is much too high.
 
Ok, I can buy branded batteries from stores just fine, I'm only asking so I know what I'm looking for.

I can't change the 12v source. How can I adapt my circuit to give me 1.5mA 9v from that 12v source?
 
A 7809 voltage regulator plus two capacitors shown on its datasheet has an output of 9V.
Buy a 7 cell 8.4V Ni-MH battery.
 
Excellent, thanks, so just to double check:

Use a 7-cell NiMH PP3 on he circuit below.

Leftmost capacitor is rated 16v, rightmost at 10v.


That should be all well and good now?

Thanks for the help.
 

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Without having the series 390 ohm current-limiting resistor that I showed you then the PP3 battery might blow up if it is discharged and you try to charge it.

The capacitors for a voltage regulator IC should be ceramic (because they work very well at high frequencies), not electrolytic (that is poor at high frequencies). The lowest voltage for a ceramic capacitor is 50V.

Why are your schematics a negative with a black background when all other schematics are a positive with a white background??
 
Ok, like this?


They look like that purely because I tend to work in either the "current flow", or "logic levels" tabs. (And that was where I print screened from)

If you prefer:
 

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Thank-you for making your schematic look normal.
It should work OK if you don't mind that an 8.4V battery will charge slowly and will never be fully charged but it will not be damaged by the extremely simple charger.
 
"Thank you" is a phrase consisting of two separate words, no need for a hyphen.

For my purpose, it should work ok. Discharging for only a short period of time, and then left on to charge for far longer periods. Thanks for the help.
 
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