Battery packs won`t charge

[Tim Mackey]

Chris, when "using finger to hide value to be calculated " if its either of the two on the bottom ( R or I ) then your finger also needs to cover the letter AND the little x, or it doesnt make sense

[christopher small]

Thanks Timbo,I`ve not used Ohms law for a couple of years now but it is slowly coming back to me,I have hundreds of resistors but can`t find the ones I want.

Eric,I did my Amateur Radio Foundation course up by you at Frodsham about 6 years ago,callsign MOMTS.helping to put up a 70 cm repeater at the top of our field behind our house for gb3vn.

Chris

[Tim Mackey]

well remember that combining two in series will result in a resistance of both values added together, and two in parallel ( of the same value ) will halve the value ( and double the wattage )

I often mix and match like this to get the value I want. 

[christopher small]

Thanks Timbo,never thought of doing that . Just found out my multimeter has 2 blown resistors,so thats another 2 to dig out,test my soldering again.

I wish I knew as much about electronics as you.Always something to fix around here.

[Brian Parker]

Nicads are robust but not bullet proof. They should be fully discharged before charging at rates above C, but if cells in a series nicad are discharged to below one volt at a higher discharge rate than C/5 there is a danger of a cell becoming reverse charged due to its low internal resistance. Not good for the cell!! (If necessary fully discharge with a suitable bulb).

The maximum charge rate for indefinite charging is C/8. The battery is fully charged when 150% has been returned, i.e. around 12 hours. A constant current charger must  be used for indefinite charging. The current remaining consant throughout the time the battery is charging/connected (use an ammeter in the circuit).

High charge rates of up to 10C are permissible (check battery spec.), providing the charge period is controlled as appropriate. A constant current charger is recommended over a constant voltage unit.

DO NOT subject a nicad to indefinite charging at a high rate unless the current is reduced to the continuous rate for the battery once charged.

 A simple 'home brew' continuous current charger can be built for a few pence using a 3 leg voltage regulator from the L78/TS78 range plus a suitable resister on the output leg, 82 ohm will give 66mA, 22 ohm for 250 mA, 10 ohm for 500mA etc. Use 5W resister for 22 and 10 ohm. Connect a diode across the input and output legs. Link the centre leg (common) to the battery side of the resister. A tantaium capacitor from 'common' to input leg and that's all. You will of course need a suitable power supply/source(PSU) at 12-15 volts. DO NOT connect the PSUground/negative to the L78 'common' but to battery only. Mount the L78 on a heat sink, the resisters will also get a little warm. Don't forget the ammeter.

All available for Maplins or RS.

[Brian Parker]

Last sentence should read All available from Maplins or RS.

[Brian Parker]

Have tried to paste sketch of the above, failed twice!

How do you paste/insert without going through the gallery?

[Former Member]

[This posting has been removed]

[christopher small]

Eric -Thanks that looks ok for me .I did build a crystal calibrator for my intermidiate exam,but that was a bit fiddly for me.It did work though.

All I want for now is to charge up my batteries.

[christopher small]

Thanks Brian-I have just seen your post.all very helpfull.

[Brian Parker]

Chris,

I'm still unable to post my sketch of the above to the site, but if you are interested then you will see from the description there are only 4 components costing under £1 in total.

The Resister/Current values in the description are using a 7805 regulator. Thisgives us our reference voltage...... V(ref)   i.e. 5Volts. The circuit has a quiescent current of 5mA.... I(q).

Then using Ohms law .......  I(out )= V(ref) / R + I(q)

The diode used( IN4001) across  the regulator has it's cathode to the 'in' leg.

Use a 1microfarad capacitor.

A PSU of 15volts or over will allow charging of single cells up to 10 cells (12volt battery). I use the high wattage resisters to keep things cool. The heatsink is a piece of aluminium bolted to the regulator and components are soldered direct, veroboard not needed due to low component count.

Two or three full charge/discharge cycles using the above circuit should restore your batteries to full capacity provided they  have not been too 'overcooked'.

[christopher small]

Brian-if this puts out 5 volts,how do i get the 7.2 for charging my 7.2  400 mAh battery pack.

Looks good,I should have all the components including the 7805.its putting them all together that is the hard part.

[Brian Parker]

Chris,

It's all in the configuration of the 7805 as a constant current source.

In my original posting I emphasised that the centre leg (common) of the 7805 is NOT connected to negative, but to the battery side of the output resister. This use of the 7805 is to provide a  constant 5 volt drop across the resister.  It is not used as a conventional voltage regulator.

I have used this circuit for charging nicads for many years for both single cells and upwards to 8 cells, even PP3 type. It is based on an old data sheet (probably from RS), It is usable from 1.2V (single cell) to 12V(10 series cells) with the 7805 and sugested PSU.

I actually use a plug board arrangement to select the required charge current from various resisters wired in series, parallel and series/parallel. Not ideal but like Topsy, "it just grew".