[SOLVED] charge batteries while in radio?

[inductor]

Hi
I have a radio, it draws a maximum of 100mA.
It runs on four D batteries or 6VDC adapter.
I use NIMH batteries in it, 2.2 amp hours.
When you plug in the adapter the batteries are bypassed. I put a jumper so that the batteries were not bypassed, this way I didnt have to take the batteries out to charge them. Eventually the adapter burnt out.

What simple components could I add between the adapter and the radio so the batteries would just get a small trickle charge? Also there are frequent power failures here so I dont want the batteries to drain out during a power outage.

 

[pranam77]

NIMh batteries usually tend to take more current during charging cycle than NIcd batteries. The adaptor which was meant to deliver 100Ma as mentioned, was forced to deliver more current which it coudnt. For charging the NIMH batteries you may construct a constant volatge charging circuit built arround LM317 or so and then connect the batteries,+ charger+ radio in parellel. So that no swicthing is required. But be sure that your charger transformer bieng used can handle the charging of the batteries in addition to the full consumption of the radio as both will be powered at a time.

OR you may experiment with different values of low value resitors in place where you connected a jumper now. The values may range anywhere from 10 - 100 Ohms 2 watt. This method of charging is not safe compared to the one expalined above. You are at your own.
Cheers

 

[inductor]

Ok
But what will stop the batteries from being drained if there is a power outage?

 

[betwixt]

The charge methods Pranam77 suggests will not drain the batteries unless current can flow backwards into the charging circuit which is unlikely. What you need to do is limit the amount of current that can flow IN to the batteries so they charge without overheating and also do not overload the charger transformer. Normally no current would flow OUT of the battery unless the radio was turned on. Do be careful not to exceed the transformer maximum current rating if you are using the radio and charging the battery at the same time as both will load it at the same time.

Brian.

 

[inductor]

Two questions.

If I make the constant voltage charging circuit what voltage should I make it?

For the simple one resistor method what if instead of a resistor I put a 6V 250mA flash light bulb across the positive line?
When batteries are low the bulb would be dim when the batteries are fully charged the bulb would be bright. Also the bulb's resistance is high when the bulb is bright so that should prevent overload of the 400mA adapter.

 

[betwixt]

If you put anything conductive across the supply all you do is waste power into it. If anything, you should wire the bulb in series with the charge feed to the battery so the bulb is brightest when the battery is discharged and gets dimmer as the battery voltage rises nearer to the charge voltage.

It will work but not as effectively as a dedicated charging circuit which would optimize the current so the battery reaches full charge as quickly as possible while staying within recommended limits then drops to trickle charge to keep it topped up.

Do not use a constant voltage charger on NiCad or NiMh cells, you risk an explosion. It's constant current you should use so the applied voltage rises as the cell recharges.

Brian.

 

[inductor]

Yes light bulb in series with the charge feed, that is what I meant.
The 6VDC adapter is rated at 400mA.
The maximum current the radio draws is 100mA.
The NiMh batteries are 2.2 amp hours.
What should the mA rating of the 6V light bulb be?

 

[betwixt]

It's difficult to give a precise answer without knowing the exact voltage and current relationship in the bulb, as you point out the resistance changes with filament temperature.
You also have to know the manufacturers recommended charge rate for the battery. If I assume the charge rate is C/10 the maximum current would be 220mA so that is what the bulb should be rated at because they are in series. In practice, the bulb will not provide constant current of course but 220mA would limit the maximum current to safe level. In a perfect situation, the charge would be 220mA into a completely discharged battery at almost 0V and still be 220mA when it had reached 6V.

Brian.

 

[inductor]

Thanks Brian
Yes charge rate is C/10. I think a bulb that is between 200 to 300mA should be ok.
If the charge rate ends up less that C/10 it will just take longer to charge thats ok as I plan to leave the adapter plugged in 24/7 and batteries only used if there is a power outage. As long as the batteries and adapter dont get hot and the batteries do eventually get charged thats fine, I will watch and see.

I think using a bulb is far from ideal but its much better than using a resistor, dont you?

 

[betwixt]

As you say - not ideal but certainly a workable solution.

Brian.

 

[inductor]

Ok
One more question which has more resistance at full brightness a 100mA bulb or a 300mA bulb?
:?:

 

[betwixt]

100mA has highest resistance. For a given voltage, the resistance has to be lower for more current to flow.

Brian.

 

[inductor]

Ok
I found bulb for 6V lantern so its actually 4.8V 300mA, it charged batteries all the way up and nothing got too warm, bulb is a little dim at all times, at full volume bulb brightens a bit when theres a loud noise like when WWV hits top of the minute.

Looks good to me........