Value of R54 for battery trickle charging..

[vk7jb]

Hello Bob & Group,

This is a very basic question and I hope someone can help.

I have an Aim4170 and I’m trying to set it up to trickle charge an internal, 9V 230mAhr NiMh battery.  

I’ve connected the battery wires to the Battery + and bat b pads on the circuit board and have attempted to calculate a value for resistor R54.  I’d like a charge current of around 20mA and according to my calculations, with a 12v supply, that should require an R54 value of 12/0.02 = roughly = 560 ohms.

The problem is, when 560 ohms is installed at R54 the measured charging current is one tenth that expected- ie around 2.4mA.

I’ve checked the supply voltage at the pads on the board and it’s indeed 12V.

Any ideas what I’ve done incorrectly in my calculation of the R54 value, or what am I not understanding?

Many thanks,

John
VK7JB

[vk7jb]

Hello All,

I'm back to answer my own question

After some thinking about this, it occurred to me that the value of R54 = Vs-Vb/I, where Vs = DC supply voltage (12V from a wall wart in my case), Vb = battery terminal voltage and I is the desired constant current for charging. I wanted a current of between 0.1 and 0.2C and my battery capacity is a nominal 230mAhr at 9.6V, so the target charging current is 20-46mA.

Plugging the numbers in to that equation gave me a value of R54 of around 100-120 ohms. I used 100 ohms. This gives me a charging current of 40mA when the battery is fully discharged (8V), 24mA when it's at 9.6V and 20mA when fully charged (around 10V). I've checked a charging cycle and the measured current values accord pretty well with the calculations - in general, the measured currents are marginally lower than the calculated.

FWIW, I've also just dropped in a female FTDI DB9 to mini-USB interface on the rear panel in place of the original DB9 serial connection. I've been able to do away with the bulky serial to USB converter cable I was using. Very easy conversion and it's working a treat.

73,
John
VK7JB