battery charger question

[Mdkanz]

i have a peak detection charger that i use to charge 6 cell nimh sub C battery packs. i connected a volt meter to it and left it on while it was charging if went up to 9.40 volts then started lowering and said it was charged at 9.24 volts would i get more voltage if i disconnected it when it said 9.40? (BTW i'm charging them a 4 amps)

 

[audioguru]

The peak voltage blip from charging six Ni-MH cells in series is so small that you might not see it on a meter. When it drops to 9.24V then the battery is way over-charged and might be damaged by high temperatures and high internal pressure.

I got this graph from Energizer's website where they have datasheets on all their batteries and have Applications Manuals:

 

[Mdkanz]

the voltages i listed were measured while the battery was charging. those were not the actual voltages of the battery.

 

[audioguru]

the voltages i listed were measured while the battery was charging. those were not the actual voltages of the battery.

The graph from Energizer Battery Company i posted shows that when the voltage peak occurs on a ni-mH cell then it is over-charging and its temperature and internal pressure are rising toward destruction.

Something is wrong with your battery charger to allow over-charging.

 

[kchriste]

Most nimh chargers terminate on negative ΔV. ie: They stop charging on the down-slope of the "hump". The 100% in the chart is the charge input of rated capacity, which I read to mean, for example, when you have been charging a 1000mAh battery for 10 hours at 100ma, you are at 100% "of rated capacity". The battery isn't fully charged at this point (100) because charging isn't 100% efficient. At least that's how I read it anyway.

 

[mneary]

The graph from Energizer Battery Company i posted shows that when the voltage peak occurs on a ni-mH cell then it is over-charging and its temperature and internal pressure are rising toward destruction.

Something is wrong with your battery charger to allow over-charging.

If it continued, destruction would certainly occur. The key is toward destruction. The charger designer obviously thinks this is a good place to stop. I agree with kchriste, this is common with fast-chargers. Others use a thermistor inside the battery pack.

 

[bountyhunter]

Audioguru is correct. Voltage inflexion is not a good choice for use in detecting EOC (end of charge point) for NIMH batteries. It works great for NI-CD, not NI-MH. To safely detect NI-MH end of charge, it is typically done by measuring delta temp/time. Problem is, NIMH is an exothermic charge reaction so the cells heat up during charging all the time, but they heat up at a faster rate when EOC is reached so you have to measure the time rate of change for temperature to catch the EOC point. Cell voltage might be used as a backup for temp detection, but I would not use it alone on NIMH.

 

[bountyhunter]

Most nimh chargers terminate on negative ΔV. ie: They stop charging on the down-slope of the "hump".

That surprises me. I worked in the development cycle of NI-MH batteries with Gates Energy (who invented them) and I distinctly remember they said that using voltage sensing to terminate charge cycle was not good for the NIMH chemistry.

maybe the charger makers don't care if you kill the batteries early in their life?

 

[bountyhunter]

Here's a link to an app note on a battery charger IC written by Fran Hoffart. Smart guy, I shared an office with him for a few years and taught him everything he knows...... He went from nat Semi to Linear Tech a while back.

https://www.electro-tech-online.com/custompdfs/2009/09/dn380f.pdf

Standalone Charge Termination

The charge termination methods used by the LTC4010
and LTC4011 utilize battery voltage and battery
temperature changes to reliably indicate when full
charge is reached as a function of the charge current
selected.

This IC has built in circuitry for both types of charge termination: cell voltage inflexion and temperature rate detection. That means you can use this IC for either NI-CD or NI-MH and safely terminate fast charge.

If you study attached figure, you will see why using the dT/dt (temp change) detector subjects the cell to less overcharge abuse: it detects right when the cell is full charged, the voltage dip occurs later when the temp has really skyrocketed and internal pressure is taking off. This is basically the same as what's shown in Audioguru's plots.

 

[kchriste]

I agree that temperature sensing is much better for battery life. Most NIMH chargers you buy (For the common AAA,AA,C,etc) terminate on negative ΔV because it is the cheapest method. Battery "packs" usually have a temp sensor built in so their chargers can use the rapid increase of ΔT to terminate the charge.