4 x 9V battery charger

Hi

I plan to use 4 x 9V batteries NiCd or NiMH (in series) to power-up my circuit which needs 12V and 36V. Each battery is rated 300mAh. If required, I can go for more expensive Lithium-ion batteries which are rated 500mAh.

1. Can I use 36V battery charging circuit to charge all 4 batteries simultaneously?

2. Will there be any difference in circuits for 3 x 12V SLA batteries and 4 x 9V NiCd batteries?

3. How much should be the charging voltage and current?

1. That's not a good idea since the two sets of batteries will likely be at a different state of (dis)charge.

2. There's significant differences between charging the two types of batteries. SLA batteries can be charged using a constant voltage. NiXX's need a constant current.

3. That depends upon the particular battery you use. Look up the info on the manufacturer's website.

A "9V" Ni-Cad or Ni-MH battery might have 6 cells and produce 6.6V to 8.4V or it might have 7 cells and produce 7.7V to 9.8V. Then four batteries will produce 26.4V to 33.6V or 30.8V to 39.2V.
A Lithium cell is 3.2V to 4.2V and three will produce 9.6V to 12.6V.
Three 12V SLA batteries will produce 36.6V to 41.4V.
So there is a range of voltages as the battery voltage runs down and the different types of batteries have different voltages.

A Ni-cad or Ni-MH cell is charged with about 1.4V at 1/10th its rated current capacity.
A Lithium cell must use a lithium charger circuit that charges at its rated charging current that might be as high as 5 times its rated current capacity and with the voltage limited to 4.20V. The charger must turn off when the charging current drops to a small current.
A 12V Lead-acid battery is charged with about 13.8V at 1/10th its rated current capacity.

crutschow said:

1. That's not a good idea since the two sets of batteries will likely be at a different state of (dis)charge.

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Thanks for your response.

Should I go for a twin 18V charger circuit, one for each pair?

Li-Ion is best choice for power density, capacity.

NiMh is cheaper. SLA are cheapest.

cost is usually inverse with ESR and proportional to Ah capacity, which affects load regulation voltage. Some cheap Chinese no spec LiPo batteries may seem to be a bargain, until you discover flaws which affect life cycle limit from temperature rise, but LiPo fom reputable sources may be best if price is right.

NiCd were low ESR but now are obsolete due to environmental concerns, with minor exceptions.

All important specs can be found here.

DO NOT buy batteries on ebay because many fake batteries are sold there:
https://www.youtube.com/watch?v=eOshOXcSkDA

I want to integrate the charging circuit with the main circuit. Regardless of the battery type, do I have to make 2 charger circuits i.e. one for (2 x 9V) set?

If you use Ni-Cad or Ni-MH batteries, I think they should be charged separately so that each one will be fully charged. Two batteries are not identical so one gets an overcharge and the other one gets a partial charge if they are charged in series.
Lithium battery cells are charged on a balanced charger that fully charges each cell separately.
I also think that 12V Lead-acid batteries should also be charged separately.

Audioguru said:

If you use Ni-Cad or Ni-MH batteries, I think they should be charged separately so that each one will be fully charged. Two batteries are not identical so one gets an overcharge and the other one gets a partial charge if they are charged in series.
Lithium battery cells are charged on a balanced charger that fully charges each cell separately.
I also think that 12V Lead-acid batteries should also be charged separately.

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In addition to what AG mentions, the initial state of charge of each cell could be different.

Therefore, batteries MUST be charged individually. Regardless of chemistry.