Can I use a PWM solar controller for charging a 24V battery from a 50V solar panel

Question

I have a 460W solar panel with 50 Voc (open-circuit voltage), and a 24V battery bank. I'm thinking of using a PWM (60A) controller to charge this battery bank.

My understanding with PWM is that it doesn't step down the solar panel voltage before applying it on the battery. If I understand this right, it would be attempting to charge the 24V battery bank from whatever the voltage the solar panels provide!

Wouldn't this large voltage difference (24V vs 50V) be harmful to the battery bank in the long run?

Answer 1

1. The solar cell won't be operating at 50V since Voc implies zero current draw. You will need a solar panel controller to effectively use the panel.
2. Applying 50V to a 24V battery with no intermediate circuitry will damage something.
3. PWM can step up or down depending on the configuration, without a datasheet there's no telling.
4. There are units out there that incorporate solar panel control (called MPPT, maximum power point tracking) and battery charging capability in one. Just make sure you match your solar cell specifications and your battery specifications.

Answer 2

Usually a PWM circuit include some Inductance to "smooth" the pulsed charging signal. And, usually also include an output current limiter. The Inductance will make the bridge between the solar cell 50V and the battery 24V. the Inductance is storing energy for the duration of the pulse and sends it back to the battery. Practically speaking it is being charged with 50V for a short time and connected back to the output for the remaining of the period, re-injecting that energy into the battery, at the battery voltage. And to smooth things out there is a decent size capacitor at the output, such that the battery receive a regulated source of current. That regulation is not ideal and could never be, but it is totally safe for your battery so long as the PWM module is correctly set to not feed the battery with more voltage than capable.

You may have to verify with a scope on your PWM circuit but still, even if it pulse the whole 50V from solar cells onto the 24V battery, the battery impedance is so low on a healthy battery that the voltage from the solar cells will go down momentarily when switched ON.

I do not see any problem with such an arrangement.

Answer 3

Presumably, you have chosen a charge controller that is designed specifically to work with a 24V battery or, is configurable to work with a 24V battery. And presumably, it is designed for/configurable for the exact battery type; AGM, Gel, LiFe, or whatever.

If it can handle 60A at 24V, that's comfortably greater than what your 460W panel can produce, so you're good there.

What's left is, somewhere in the specs or data sheet for the controller, it should tell you the maximum Voc for the solar array that the controller can withstand. If that number is greater than 50V, then you should be good to go.

Answer 4

You haven't give the panel maximum power point voltage, short circuit current, or operating temperature, but you seem to have the correct panel voltage and battery voltage*

A typical 72-cell solar panel will be about 48V open-circuit, and about 40V at maximum power point.

A typical 24V battery will charge at about 28V.

A typical solar cell installation will drop about 12V just in the wiring.

28+12 = 40: a typical 72-cell solar panel is the right size for getting the maximum power into a 48V battery system.

A DC system designed for a "48V battery" using "50V solar panels" uses a "PWM" charge controller to disconnect the battery from the solar panels when the battery is full. The voltage is matched by the ordinary characteristics of the panel, wiring, and battery.

A "50V" solar panel is unusual, strange, or they are just trying to make it easy to understand. Actual solar panels are made from solar cells, which have a voltage which is not a nice number. But a panel that give 47.2V open circuit at 80F, will give more volts at 32F, so the headline-number can be anything you want it to be unless you are selling to people who are comparing different panels.

If you aren't bringing 12 Ampere down from your roof to a battery room, you may not be loosing 12V -- but that's only 1 Ohm, and if you aren't bringing wires down from your roof, you may be using thinner wires, and a lot is lost in the connections anyway -- so it's easy to loose that kind of voltage in the system.

If, instead of a DC system with a PWM charge controller, you have a MPPT DC/DC converter, or grid-connect AC system with an inverter, you don't need to match the battery to the panel to get maximum efficiency or to protect the load. You can use 60-cell panels, or 120-cell panels, and the MPPT or grid-tie inverter will match for you.

Since almost all solar panel systems at present are grid-tie inverter systems installed on houses, most installers doing home solar-panel systems don't know anything about matching panel voltage to battery voltage, or matching load to battery capacity and panel capacity.

*The worst case is that you have cheap, unapproved components and that your supplier has lied to you: I've seen inside a 'charge controller' which was nothing but wire connecting the solar terminals to the charge terminals. It can be like buying USB thumb drives or memory cards on the internet. I do not know what you actually have.