# Simple solar charging circuit for two 18V Lithium Ion batteries

#### guyburns

Feb 26, 2022
2
I have 36 volt equipment that uses two 18 volt (5 cell), Li-Ion batteries, 6AH each. I'll be a long way from cars and power points when I use this equipment, so I've settled on a solar recharging. I want to keep the recharging as simple and as efficient as possible i.e. minimal electronics so that losses are kept to a minimum.

I'll recharge the Li-ion batteries, in parallel, to 20.25 volts, and will continue doing so at home for the life of the equipment. According to Table 3 on this web page, charging to 4.05 volts/cell will give me 3 times the number of discharge cycles over the life of the batteries, at 80-85% capacity.

I do not intend having a charging cut off for the 18V batteries, except by manual disconnection. Experiments at home have shown me that the charge current naturally falls to zero as the battery voltage rises to 20.25 volts, well below the maximum recommended charge voltage (20.8). If no current is going in, it seems to me there can be no overcharging.

Here's my idea (see attachment):

1. Use this solar panel, max power at 17.2 volts, 2.3 Amps.

2. Connect the solar panel to a 12volt, 30AH lead acid battery. No controller, just a simple cut-off circuit if the voltage ever reaches 14.4 volts

3. Connect the battery to a DC-DC converter, voltage set to 20.25 volts (5 x 4.05 volts), current set to 2A

4. Connect the output of the converter to the two 18 volt batteries in parallel.

5. When the sun goes down, I disconnect the setup. Assuming I've had >8 hours of sunshine that day, the 18-volt batteries should be 85% charged, and the 12v battery should be well charged.

Can anyone see any problems with this setup?

#### Bluejets

Oct 5, 2014
5,995
Converters have no limiting factors required for charging (or discharging) such cells.
If you have access to a 12v car battery, connect your solar to there by all means but the Li-ion best charged using an appropriate charger.
There are r/c aircraft chargers available that will safely charge up to 6 cells from a 12v supply, not all that expensive.
Good units can be had for around the AUD\$80 mark.

#### guyburns

Feb 26, 2022
2
Thanks for the suggestions. Just one correction though – the converter I linked to has two pots for setting current limit and voltage limit. When I begin testing, I'll set those to 20.25 volts and 2 amps.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,774
A charger worth its name has a voltage dependent current limit. Typically you start charging with a comparatively high current. then reduce charge current once the battery reaches it's fully charged state. Evaluating the "fully charged" state depends on battery chemistry and (ideally) environmental conditions, i.e. temperature.
While it is possible to charge a battery from a voltage and current limited DC power source, this is far from ideal and can shorten the lifetime of the battery drastically. Worst case scenario: exploding battery.
I second @Bluejets ' recommendation.

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