Charging GP340 manually

[kg2451]

Hello,

I have a question directed to Will or anyone else with experience of manually recharging radio batteries.

I have a UHF GP340 with a 7.5V NiMH battery, which I believe is 1200 mAh (HNN9008A). I do not have the charger and am looking to charge the battery manually with a power supply.

The power supply I have to work with is voltage selectable between 1.5/3/4.5/6/7.5/9/12V, and the current is fixed at 300mA.

What I was wondering is what would be the best voltage to use for this, how many hours should I let it charge for, and what resistor or other components should I use for limiting current?

I believe I should be charging at 120 mA, however my power source only outputs 300mA, which hopefully a resistor can take care of. Also if I need a resistor, where would that be placed in this circuit, between negative terminals?

I'm thinking if I chose 12V than a 37 or 38 ohm resister might do it if:
Limiting Resistance = ( Supply Voltage - Battery Voltage ) / Desired Current
37.5 ohms = ( 12V - 7.5V ) / 120 mA


I've heard that the last 80% of a charge for some batteries is important, in which case should I not let it get to 80% using this method, or is there something I should change like dropping the current with more resistors at a particular time? I just don't want my battery becoming shrapnel.

Also just to verify on the HNN9008A with the back facing you and with the four charge contacts on the bottom, the negative contact is on the far left (1), and the positive is on the far right (4). And from the power supply the positive lead goes to the positive contact, and negative goes to negative.

Thank you for your help.

[Will]

I believe I should be charging at 120 mA, however my power source only outputs 300mA, which hopefully a resistor can take care of. Also if I need a resistor, where would that be placed in this circuit, between negative terminals?

I'm thinking if I chose 12V then a 37 or 38 ohm resister might do it if:
Limiting Resistance = ( Supply Voltage - Battery Voltage ) / Desired Current
37.5 ohms = ( 12V - 7.5V ) / 120 mA

.

31.6 ohms = (12v- 8.2V)/.120 one watt resistor to be safe..

You forgot the isolation diode in the battery that keeps the battery from being shorted out from the keys in your pocket.

I think you have it correct, just put the resistor in series with the + lead, to the charger input and - lead on the - side of the battery..

There are four contacts on the outside of the battery, one is battery -, one is the tempature sense usally measures 10k ohms to the minus terminal, and one is the capacity resistor, from 2 to 10 k ohms to -, and the last one is the charger input thru a isolation diode to the + side of the battery.

And a way of holding the two leads on the battery for 12 to 16 hours.

[kg2451]

Great thank you very much for your help. Yeah I forgot to factor the isolation diode, which makes enough of a difference.

Ok so just put the resistor between the positive charger lead and the charger input (positive) battery contact, like this:




I'll go for a one watt resistor, but do you think I could get away with a 1/2 watt if needed, since the needed wattage is about 0.456 W?

Also if I couldn't find a 31.6 ohm resistor locally, can I combine them in series safely? And if I could how does the wattage ratting work for resistors in series; does it disperse with more resistors, or would each one still need to be rated one watt?

Is there a range of safe resistance that you would recommend if I couldn't mach 31.6 ohms exactly (for example 30.5 - 32.5)? I'm wondering because I can easily get this 22 ohm 1/2W resistor locally without shipping costs, and I have two 4.7 ohm 1/4 watt resistors lying around; would 31.4 ohms combined work or does it need to be exact? Or could I just use this 33 ohm 1/2W resistor, which I think would bring the charging current to 115 mA?


One last thing, since I'm basically charging blindly and won't know when the battery is fully charged, is there a particular magic number of hours you would advise I should stop charging the two year old 1200 mAh battery from nearly depleted, instead of a range? I want to get the best life cycle for the battery and wouldn't want to overcharge it, but perhaps undercharging is not always good either.

Thank you very much for your help, I really appreciate it.

[Will]

Looks like you jave got the hang of it.

The 22 +4.7 + 4.7 ohm combination is right in the ballpark.

Depending on the batteries RATED capacity, ie; 1200mahr, can charge nicley at 110 to 120 ma for about 12 to 16 hours. There is a fudge factor that does increase the time as shown in the example.

The best way to see when the battery is very close to fully charged, is to monitor the terminal voltage for the Delta V.. Delta V is the point at which the terminal voltage begins to decrease. Delta V is used in the rapid higher than 1/5 C chargers.

Most batteries are very happy at a 1/10 charge rate. 1c, or mahr capacity, devided by the accual charge curent applied. 1c = 1200mahr, divided by the accual rate of 120 ma is 1/10c.

[kg2451]

Great, thank you for your help, that sounds like the best method to see when it is fully charged.

One last question: what would happen if instead of 12V, I set my 300mA power source to 9V? Would that negatively affect the charging cycle or charging time of the battery at all, or would it just reduce the resistor I need to use? I only ask in the event I can't easily get a 31.6 ohm resistor.

By my calculation I would need a 6.6 ohm resister if this was possible:
6.6 ohms = ( 9V - 8.2V ) / 120 mA

And I believe the wattage used would be only 0.096 W. I ask because I already have two 3.3 ohm 1/4 watt resistors on hand that I could use if I'm unable to find suitable ones locally.

[kg2451]

Sorry I know I said that would be my last question, but I have one more in addition to that one.

I was talking with someone with an electrical background and they are concerned that I may not have the correct power supply to do this, although they have never done this before themselves.


You have said before that battery chargers use constant current, not voltage, to charge NiMH batteries. You also said to use a "current limited power source" to do so. The person I talked to indicated they believe there are two types of power sources: voltage limited sources, and current limited sources.

The power source I am using is basically just a class 2 transformer with a voltage selectable switch on it (this item). What I was wondering is if this resistor I am adding is making my source a current limited source suitable for charging, or if there is something else I need to do to make this suitable.

I was assuming that the resistor was all I needed to safely do this, but I just want to verify that I don't need a specific type of power source to begin with.

[Will]

You have said before that battery chargers use constant current, not voltage, to charge NiMH batteries. You also said to use a "current limited power source" to do so. The person I talked to indicated they believe there are two types of power sources: voltage limited sources, and current limited sources.

What I was wondering is if this resistor I am adding is making my source a current limited source suitable for charging, or if there is something else I need to do to make this suitable.

I was assuming that the resistor was all I needed to safely do this, but I just want to verify that I don't need a specific type of power source to begin with.

Yes the resistor does limit the current to a safe value to charge the battery. The "power supply" is basically a fairly constant voltage source.
The one you listed is one we have also used, it is a voltage regulated power supply. I would stick with the 12 volt setting.

The same circuit you show can be connected to a car battery, again the resistor limits the current to a safe value.