# DC-DC booster to charge an EV battery from a solar cell?

Can a DC booster provide good current to charge a 58V ebike battery from an 18V solar solar cell?

The 20A DC booster has LVC (low voltage cutout) for the input from the panel, i.e. when it goes under 17V, and CC (constant current) and CV (constant voltage) adjustment so that I can put the voltage at 58V.

The problem is the constant current. If I set the CC x CV product to 200W and the solar panel is only providing 20-75 watts variably, will the CC circuit stop the current and provide zero energy?

What about protecting the solar panel from an overload? A 58V 50A battery hooked up to a solar cell through a DC booster requires protection?

• What is "good current"? What is "LVC"? What is "panel" you refer to? If you are referring to equipment then you should link the manual or data sheet. You talk about constant current setting being 200 watts (the constant current. If I set it at 200W) and that doesn't make sense because current is measured in amps. Commented Feb 23, 2023 at 11:38
• @Andy_aka by good current I meant without very much inefficiency and loss. LVC is a low voltage cut protection. Yes sry i meant watts. Commented Feb 23, 2023 at 15:27
• DC boost varies the volts so amps on one side is less than other, however watts are the same. Commented Feb 23, 2023 at 15:30
• @bandybabboon Please check my edits and ensure that they say what you intended. Try to explain terms that may be unfamiliar. CC and CV are probably OK. LVC peraps less so. I've added expnsions in each case. See also my comment change re 200 Watt. Commented Feb 24, 2023 at 11:20
• If possible please provide a link to the panel spec. Also try to provide maximum available details. Is that 18V Vmp or Voc or ... . What is the current spec. Power spec is usually Pmp = Vmp x Imp. Voc is usually about 15 - 20% higher. Commented Feb 24, 2023 at 11:22

Most of these converters - without further information - are designed to convert power from some endless power source to a certain voltage to feed a load. Their controllers will prioritize getting the right voltage to the output, ignoring the input. That isn't what you want here.

If the output wants more power than the input can give, most likely the converter tries to take more and more current from the input, completely oblivious to the fact it causes the voltage to drop, until the voltage is too low and the converter shuts off. You said it has a UVLO feature so it might shut off before the controller loses power, and it might cycle on and off repeatedly.

If the output wants less power than the input can give, it probably works okay. At least until the first cloud passes over the panel.

How does it know how much power the output wants? That's based on the CC/CV settings you give, and the load impedance. Note that the CC setting times the CV setting is the maximum power it might try to output, but probably not the actual power at any given time. E.g. if you set it to 60V/0.6A, it might only output 60V/0.3A, and then if you wanted it to continue delivering 36W you would have to increase the CV slightly which would increase the current (not the CC setting) as well.

Solar panels often have two voltage ratings - the open-circuit (maximum) voltage and the maximum-power-point voltage, which is quite a lot lower, perhaps 2/3 of the open-circuit voltage. Puffafish pointed out that if 18V is the maximum voltage of your solar panel, you'll only be able to get a tiny trickle of current before the voltage drops below 17V and triggers the UVLO. The maximum voltage (open circuit voltage) of a solar panel is quite a lot higher than the useful voltage, which it drops down to when you start using any significant current. If 18V is the MPP voltage, you're okay (at least in good sunlight). Otherwise, if you can decrease the UVLO voltage or turn off the UVLO feature, you're okay.

So it could work in a pinch, but if you want this to happen automatically without babysitting it then you want a different converter designed for solar use, probably labelled with the keyword "MPPT". The booster is the same; the control algorithm is different. Instead of prioritizing keeping the output voltage and current steady, an MPPT controller prioritizes getting as much power as possible from the input.

If you're not interested in DIY circuitry, you can buy a fully integrated "MPPT charge controller" designed for your type of battery (probably). Just hook up the panel to one set of terminals, the battery to another set of terminals, there may be some settings to program (type of battery, maximum charging rate) and bob's yer uncle - let the computer do the rest.

• Thanks a lot that's an especially easy to understand run through, I can learn a lot and it makes a lot of sense. Commented Feb 25, 2023 at 18:23

Can we charge a 58V battery from an 18V solar cell: Yes. Boosting DC voltage is straight forward in theory.

Will the CC circuit stop/provide zero energy: maybe depends on the product and setup, you’ll need to check the datasheet as to what it does in this situation.

What about protecting the solar panel: Always protect your solar panels. A well designed DCDC should not let the panel be exposed to the 58V out, but a diode on the output of the panel is the very least requirement.

And now to answer what you’re really asking: how do you charge a 58V battery from a 18V solar panel.

First things to note: your panel only give 18V in ideal situations in an open circuit. So the voltage out of the panel cannot be relied upon. The power of the panel is more interesting to us anyway. Voltage can always be changed via a DCDC, but power is defined by what the solar panel can provide. You should therefore set the CC to be a current based on what the solar panel can provide. This setting will vary as the solar panel is exposed to different light levels. How you do this will require monitoring of the solar panel, and result in you basically doing a maximum power point tracking (often called MPPT). This is not a straightforward activity, but it is possible. Most MPPT in the market are designed for charging of lead acid battiers, of 12V or 24V, but others exist and different votlage outputs can be designed.

TLDR: yes it is possible to charge your battery, but doing so is non-trivial. And yes the solar panel will need protection, at least with a diode to stop current flowing into the battery.

• When you say MPPT, if the solar panel is rated at 100W maximum, what happens if I just set the output CC and CV at 100W equivalent? Why is that non-trivial? Commented Feb 23, 2023 at 17:56
• @bandybabboon because the maximum output is 100W, that does not mean you GET 100W at any particular point. You'd be lucky to getting 50W most of the time. So the design needs to be able to see what the real maximum power is, and adjust the current value accordingly. Commented Feb 24, 2023 at 7:44

Another way to is to place a smaller battery, 12/24v before the DC booster, to work as a large capacitor for CC, and to use a commonly available 12/24v solar controller to connect it to the solar cell and the DC booster. That way I'd have a 12V and 58v rail in the car.

Can keep the cells above 3.6V and to start charging again at 3.8V per cell on the 12v battery because that's a storage voltage.