Constant voltage NiMH chraging - why not?

Hi, I am thinking too much and found another heretic idea: why not to charge NiMH batteries by constant voltage source? It is said fully charged NiMH cell has 1.4V. When discharging it quickly drops to less than 1.3V and about 80% capacity the cell is over 1.2V. So I thought charging the cell to 1.3V would prevent possible overcharge but still charge the cell to about 80%. Yet everyone on internet says it is bad. Noone charges NiMH this way, using complicated ways of dV or dT to detect full charge. Another way is using constant current ~0.1C (or less). It is easy but you need to know capacity if you want to apply it. What if I want to charge different sizes of batteries easily? What about old batteries which lost some (unknown) amount of capacity? Charging to constant voltage looks so easy and safe at expense of small fraction of capacity - do you know where is the catch?

http://batteryuniversity.com/learn/ appears to be the most comprehensive and reliable source of advice on charging batteries of all types. NiMH here.

I have read this. I did not find any definitive reason. But maybe I overlooked it - can you point me closer if you know it?

I have two motivations for this.

1) Idea of battery backup for my future project. It will run from 2.6 (or 3.9) V, batteries connected in parallel with power source recharging to exactly working voltage when power is available. If power fails batteries will provide the power. Sooo easy. (I know the voltage when running on batteries will decrease but it is nothing AVR controlled something cannot handle.)

2) I got a "dead" NiMH battery pack. Batteries are partially discharged (around 1.2V). I would like to have a way to safely charge and discharge them to estimate if it is worth trying to salvage some of them, before I make more clever circuit that will do it for me including logging data to get more detailed knowledge about their health. Only safe way I know that is easy is trickle charge. 0.05C constant current, BUT I don't know C (2200mAh nominal but how much is left)? Let say it may be worth keeping them (for backup purposes) if they have more than 400 mAh. For this 0.05C = 20mA. But than if there is "healthy" cell it will take 150 hours to charge fully (66% effectivity Battery University says) which is over 6 days...

The reason why slow charging at 0.1C is recommended is precisely because it doesn't matter if you leave the battery on charge for ages. Overcharging at that rate is a good idea, it balances the cells.

But the capacity of the cells is how much you can get out not how much you can put in. So if they're 2200mAh nominal charge at about 200mA for 14-16 hours then discharge (to 0.9V/cell) at 0.5C and time the discharge to find the capacity.

Steve

The problem with constant voltage charging is when you have several cells in series and one is dying, excess voltage then gets applied to all the good cells, ruining them.

So you normally have several phases to charging, the first can be current limited, but with a very conservative constant voltage, the idea is to do most of the charging without risking overcharge if there's some imbalance (typically cells capacity varies in a batch). Then you back off to much smaller currents to complete the charge - its a non-ideal way to balance the charge by overcharging the cells of lower capacity but at a safe rate (they just get warm).

If you charge cells just in parallel, its easy, you can apply a constant voltage and only bad cells will suffer - the cells cannot interact with each other. So each cell can be charged to its full capacity (you don't get to know what the capacities are, nor if some cells are going bad though)

With Lithium cells a balancing charger is used that has wires to all the cells in a series string so that individual voltages can be sensed - this allows charging at the highest safe rate and identification of bad cells - its more expensive and better, but required for lithium cells which catch fire if abused.

Some cell chemistries are not very suitable for constant voltage charging as the voltage curve is too flat, and variations in voltage between cells can be larger than the change in voltage as they are charged... NiCd was like this, but NiMH have a very definitely slope.