Battery Chargers - principle of operation

[Tiberius]

AFAIUI the general battery charger is a power supply with both a current and a voltage limit. Ie. if the battery is low, there is a set maximum current that it will deliver, and then as the battery voltage rises it will reach a set voltage above which the charger won't let it go.

That's the basics of a lead acid charger, and there are variations on what they do when the charged voltage is reached, depending on what the application is. Some turn off and some stay there keeping it at the top voltage.

For NiCd and NiMH the current limit with a low battery can be replaced by sensing the temperature. The charger can then supply larger currents, sometimes in pulses, up to the point at which the cells get warm. This enables a faster charge. When the top voltaqe is reached, the battery stops taking current. The charger may just maintaining this voltage or it may switch to a different mode in which it puts in a low trickle charge, either as a continuous low current or as pulses.

There are variations that look at details of the voltage behaviour when the current is pulsed to decide how much current to deliver and when the battery is charged. There is also the simple and crude method of just suppling a lowish current for a long time and let the user disconnect it after some hours, but they are not applicable here.

For Lithium, I suspect the charger is back to the earlier principle of a current limit and a voltage limit and the management system that's built into the battery controls the current and decides when it is charged.

Can anyone confirm that rough summary, or correct me please? Secondly, what are the top voltages for NiMH and for Li-Ion?

Nick

 

[flecc]

For NiMh, the voltage is usually 1.8 to 2 volts per cell, typically around 57 volts for a 30 cell 36 volt battery.

For the common 3.7 volt Li-ion cells, 4 to 4.2 volts per cell, typically about 41 volts on a 10 cell 37 volt battery.
.

 

[PaulC]

From memory, I believe that Nicad and NiMH chargers use a constant-current approach.
Cheap chargers just have a timer in them and run for, say 15 hours, inputting a constant amount of current and then stop. Hopefully the battery is fully charged but if it was only half-discharged at the start of the charging cycle, then it will be overcharged. This approach works well for the likes of tradesmen with 2 battery packs for a drill where they run 1 pack to fully discharged before switching to the other pack and placing the discharged pack in their charger.
More expensive smart chargers are paired with their battery packs - the battery pack has a temperature sensor in it which feds back the temperature behaviour of the battery pack during charging to the charger. When a Nicad or NiMH battery pack reaches full charge, then a temperature spike occurs inside the pack. The charger sees this spike and knows to stop charging the battery pack (voltage with Nicads and NiMH is not as reliable an indicator of full charge as it is with SLA batteries).
The above is why when a smart NiMH charger dies, care must be taken in selecting a replacement charger.
Paul

 

[Tiberius]

Hi Paul,

I've used that very simple approach with small NiMH cells embedded in tiny gadgets where the battery is there for backup or because its tidier than wires. Its fine where the battery is not being used anywhere near its limit and there is plenty of time available for charging. I think the norm is to charge at 1/5 C, so a full charge takes 5 hours and the battery doesn't overheat when its left on the charger. I wouldn't want to do that with bigger cells, though.

Nick

 

[Tiberius]

For NiMh, the voltage is usually 1.8 to 2 volts per cell, typically around 57 volts for a 30 cell 36 volt battery.

For the common 3.7 volt Li-ion cells, 4 to 4.2 volts per cell, typically about 41 volts on a 10 cell 37 volt battery.
.

Thanks flecc,

Those voltages are surprisingly high for the NiMH. My Torq Li-Ion is indeed, when fully charged and open circuit, 41.8 V.

But 1.8 to 2 V per NiMH cell sounds high; at that voltage does not something have to be done to prevent the cell taking more current? Is that a safe voltage to apply permanently, or is it the peak of a trickle charging pulse?

The discharge curves I 've seen for a 36V NiMH pack (I haven't worked out how to embed a link) start at 39 V and drop rapidly to then spend most of the discharge between 38 and 37 V. Ie, 1.25 normal operating V. I realise that doesn't tell us the top charging voltage, though.

Nick

 

[flecc]

NiMh need a very high charging voltage to get it done in a reasonable time since charging is in fact electrolysis, in the earliest crudest form of these, of water, to turn it into hydrogen and oxygen, so it bears no relation to the fully charged cell voltage.

They are rather odd batteries in that they are really a form of fuel cell, just a recycling one using electricity as their "fuel" to produce hydrogen which is then used fuel cell fashion to create electricity and the water ready for the next electrolysis, rather than the fuel cell using a fuel like methane or hydrogen for a single cycle.

It always amuses me to hear the lithium advocates refer to NiMh as old technology, when in fact fuel cells are the next technology and they've been doing it for years!
.

 

[Tiberius]

After a bit more reading around, I ought to add a little more.

The thermistor in NiMH packs may not be so much for setting the maximum charging current by limiting temperature, as I suggested, but more for finding the end of charge by detecting the rate of rise of temperature that occurs then.

When being charged, the actual voltage across the NiMH cells is 1.4 to 1.6 V. The 1.8 V per cell value is used as a stop signal to terminate the charging process if the temperature rise method is not used or fails.

This applies to steady current chargers. Charging with high current pulses may be different.

Nick

 

[flecc]

That's correct Nick. In fact I read your original post as meaning that anyway, but clarifying is always good since I may have been auto-interpreting.
.