Terminal voltage sensing

[Anon_LG]

I want to charge 1.2 Volt NiMh rechargeable batteries from some PV panels.

Could someone suggest how I could sense terminal voltage across the battery and use it to create an overcharge preventative system.

 

[Gryd3]

I want to charge 1.2 Volt NiMh rechargeable batteries from some PV panels.

Could someone suggest how I could sense terminal voltage across the battery and use it to create an overcharge preventative system.

Run the terminal voltage across a voltage divider, and compare to the voltage drop across a zener diode

Edit: There are other ways to generate a reference voltage. This was the first to pop in my head.

 

[Anon_LG]

Please could I have a diagram?

 

[KrisBlueNZ]

Terminal voltage isn't a reliable way to detect overcharge. At least, not for NiMH. There's a prescribed charging sequence that includes end-of-charge detection, which is best done by watching for a temperature rise. Anything that cuts charge based on terminal voltage is going to be rough at best.

 

[BobK]

Use a trickle charger at no more than 0.05C and you will be safe.

Bob

 

[Gryd3]

http://www.talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html

Charging these things quicker appears to either be a blind timer, or require some feedback regarding heat and current. Trickle charging looks like a simple bet, but many of the simple circuits I have seen require the battery to be discharged first before attempting a faster charge.
Thank you to both Kris and Bobk, as I looked only partially at the questions and decided a comparator and voltage reference were desired. The big picture however requires more work.

 

[Arouse1973]

Fast type battery chargers that are used for power tools as an example generally have two types of sensing. The first is negative delta voltage detection, when the battery is near fully charged the battery voltage drops slightly and this is what is sensed initially. It can be a bit unreliable at times as this voltage is a lot smaller or it isn't easily detected with out false triggering as compared to NiCDs and so a second method as Kris described is delta temperature rise. The temperature sensor is normally placed inside the battery pack between the battery cells. Some of the more expensive chargers might have a timer built in also.

Trickle charging is the only other option if you don't have the means to sense the termination any other way. But don't leave them on charge indefinitely as you may over charge them which NiMHs hate and it also shortens their life. As Bob mentioned 0.05C is good but you need to gauge how discharged they are first to work out the charging time you don't want to take them off charge too soon or leave them on forever and risk over charging them.

Discharge them down to no less than 0.9 V per cell. Then trickle charge them for the calculated amount of time. You can use a higher charge rate but you will have to de disciplined in turning them off after the correct amount of time. You could say charge a fully discharge battery at 0.1C for 8 hours and then trickle charge for the remaining amount of the calculated time. The best option personally is to purchase a charger designed for NiMH batteries. This way they can be charged in an hour or less and the risk of damaging the batteries which are expensive is minimised.

Adam

 

[chopnhack]

Nothing to add re: sensing V, but I read in a unrelated article, when batteries approach full charge, current decreases to nearly 0. Maybe it would be easier to sense when there is no current flow (or a predetermined lower level of current flow) as an endpoint to the charge cycle.

 

[Gryd3]

Nothing to add re: sensing V, but I read in a unrelated article, when batteries approach full charge, current decreases to nearly 0. Maybe it would be easier to sense when there is no current flow (or a predetermined lower level of current flow) as an endpoint to the charge cycle.

Current would only be zero if your supply was a constant voltage source. This would not apply to a constant current source.
Many of the documents I have read regarding charging NiMh and NiCd all reference specific current supplies expressed as a fraction of the batteries' amp hour capacity.

 

[KrisBlueNZ]

Nothing to add re: sensing V, but I read in a unrelated article, when batteries approach full charge, current decreases to nearly 0. Maybe it would be easier to sense when there is no current flow (or a predetermined lower level of current flow) as an endpoint to the charge cycle.

That only applies when the battery is being charged from a fixed or limited voltage. This method is often used with lead-acid batteries and is called constant-current/constant-voltage charging. The charger limits both its output voltage and its output current.

During the main charge time, the charger limits the current to a preset value, and charges the battery at a constant current. As the terminal voltage approaches the charger's preset voltage limit, the charger's output voltage stops increasing and the current tapers off; this is the constant voltage part of the charge. The battery can remain on constant voltage, and it draws a small trickle charge and remains fully topped up.

Not applicable to NiMH.

 

[Arouse1973]

Only a constant voltage equivalent to the terminal voltage of a fully charged cell or battery pack, not just any old fixed voltage. Do you think we need to be a bit clearer about this? Otherwise people might think they can connect a battery straight across a power supply that might be set a lot higher than their battery voltage.

Thanks
Adam

 

[BobK]

AFAIK the constant voltage method of charge termination works only for lead / acid and Lithium batteries.

Bob