One option is a charger that stops when the battery pack reaches 12 or 13V. To start charging again, the voltage must drop to 10 or 11V. This might be done with a window comparator consisting of two op amps (or one op amp with hysteresis). The op amps drive a transistor or mosfet.
Or a charger that is simply regulated to 12.5 V. The battery pack only goes that high and no higher. Install an inline resistor (or biased transistor), whose current is adjusted to deliver safe charge rate. In fact a simple zener regulator might be sufficient. It depends on how quickly you want to charge the batteries, and how often a blackout hits. It also depends on the supply voltage. The zener diode idea could be adapted to your schematic #2. It is feasible to put a resistor inline with the zener, to reduce accidental overmuch current through it.
So is it enough to connect the zener (in series with a series resistor) in the second schematic in parallel to the battery?
This is feasible only if we're talking about a power supply which is not much greater than 12V. That allows you to use low ohm resistors, and possibly a low-Watt zener.
We add the zener diode to your schematic #2:
The simulated battery pack consists of a voltage source and capacitor. Notice the battery pack begins at 9V. As it reaches 12V the zener draws away excess current.
Real zener diodes can be off-spec. You need to experiment with values, in order to find the best voltage for maintaining your battery pack. It's a question of what voltage is optimum (whether 12V or 12.5 or 13, etc.). In other words, what is your battery's resting voltage in a full state of charge? It's hard to be sure.
Thanks for the simulation!
Ok I see you have an input voltage of 14v, obviously to compensate for the diodes drop. I could include a small lm317 regulator to supply this voltage, maybe this will be adequate.
What is the charging current for the batteries with this 33R, is it 128mA? Maybe one could do this less to prevent overheating of the batteries, as no fast charging is needed. Any estimation for other values of resistors?
Do you think that it would be quite safe to build this, in the sense that the ups will be left unattended in a closed room for months maybe.
I do not want the batteries to blow up or leak, this would be quite a disaster and make things messy to clean from all these chemicals!
PS. I have some 2.7V 400F supercaps. What if I connect 5 of them in series and charge them up to 2.4v?
If you plan to let the battery pack go months without maintenance, then you should consider getting a smart charger, designed for a 12V nimh battery pack.
I've been exploring the abilities of H-bridges and half-bridges. By turning transistors on and off in sequence, you can charge each cell, one at a time. This should also provide a means for you to to read its voltage and charge rate.
Switching can be done by a 4017 IC (decade counter), since you have 10 cells. My supply voltage is 5V because the inverter gates are fixed at 5V. You need to adjust values to work with your power supply.
I tried simulations with various arrangements of NPN and PNP. This schematic seemed to work properly.
Thanks!
I am sure this schematic has something ingenious in it, but I need to ask a few things.
1. Are the batteries charged fully one at a time, or does the charging takes place in sequence with one pulse for each battery (i.e. one charge pulse for battery B1, then go to the next battery B2 and send one pulse, then B3 and so on, untill all batteries are charged simultaneously.
2. Can I connect the load at B1+ and B2- and leave this charging circuit connected as well?
3. The inverter gates are 5v, but what should the VCC be 12v or 1.2v? Also, since the output of the inverters is 5v, shouldn't I redice it to 1.2v (with a simple voltage divider maybe)?
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