ahayes avatar image
ahayes asked

How to determine the time it will take to charge a battery with an MPPT or PWM charge controller


I am developing a solar battery system to power actuators in remote areas.

I have investigated both MPPT (SmartSolar) and PWM (BlueSolar) charge controllers for this application. I have a few questions concerning exactly how these work and what settings are possible. My objective is to determine how long it will take to charge my chosen batteries with my chosen solar panels.

It is my understanding that the most common effective method for recharging lead acid batteries is to use 3 phases – bulk, absorption and float. In the bulk phase, the maximum amount of current available from the panels is supplied to the batteries. In the absorption phase, a constant voltage is maintained whilst the current is progressively stepped down. Finally, in the float phase, a small current is supplied at a constant voltage to overcome self-discharge. An MPPT charge controller uses a DC-DC converter to match the batteries voltage during the bulk phase, increasing the current to ensure all the power available from the panels is supplied to the batteries. It then uses pulse width modulation to step down the current progressively during the absorption phase. A PWM charge controller supplies the maximum current from the panels during the bulk phase and, like the MPPT charge controller, uses pulse width modulation during the absorption phase. The charge controller switches from the bulk phase to the absorption phase at ~80% fully charged and sets a timer for the absorption phase based on the initial voltage of the battery before charging began. Is this explanation correct?

For lithium ion batteries, it is my understanding that the absorption phase is not required and that they can be charged entirely within the bulk phase with charging ceasing at a set voltage limit. Is this correct?

Finally, in direct reference to the SmartSolar and BlueSolar products, can an output current limit be set? This may be useful in a situation where the recommended charging current of a battery is less than the output current from the solar panel.

Thank you in advance for your advice

MPPT Controllersbattery chargingLithium Batterylead-acidBlueSolar PWM
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1 Answer
JohnC avatar image
JohnC answered ·

Hi @ahayes

PWM is a little old-hat nowadays, but still finds a place with (say) camping systems running small loads. Cheap and 'expendable' even if damaged, left in the rain, etc.

Victron's 'Bluesolar' name covers both types, the Smartsolar mppt units are bluetooth equipped.

Output current limit can be set, but most pb batts will self-limit below critical levels in a typical solar day. And it's not needed to adjust for solar output at the lower end to make them work.. they know, and just produce the power that's available to them.

If you want, expound upon what your project entails, and we can take a look. No point reinventing the wheel, hey..

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ahayes avatar image ahayes commented ·

Hi John,

Thank you for your answer.

The company I work for produces electrical actuators, devices used to operate valves. I'm a process engineering intern with them. My project is to develop a range of solar battery systems to power their entire range of actuators as well as some associated control devices (flowmeters) and telemetry (RTUs).

The chosen RTU has a sleep mode, so the load will be very small most of the time. The batteries will be a backup system used on days when sufficient sunlight is not available. As such, power from the solar panels will be routed to the load whilst any excess will be sent to the batteries. It's this link between the panels and the batteries where I intend to use a charge controller.

I made my original post because this is my first time working with this sort of technology. I wanted to make sure my understanding was correct so I could correctly determine approximently how long it would take to charge the batteries and thus correctly size the system.

Thank you again in advance for your advice

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JohnC avatar image JohnC ♦ ahayes commented ·

Ok, so you're talking 'industrial' grade kit. A Victron mppt would be your best choice (perhaps your only real choice).

Try this System Planner:

It's a sort of interactive thing, so you might find yourself going back'n'forth through it as you change the sliders around. But it'll give you a fair idea of the inputs required to determine your needs.

It will recommend batt size in Li terms, and might use 100% drawdown to calculate that. Pb's I'd perhaps multiply by 3-4x to size them. Days of Autonomy you'll need to get your head around, and the Planner should also indicate the difference between seasons, and why panel specs can come with grain'o'salt doubts when they don't produce full power all day.

You'll also need to determine batt V, and any ac needs will then require an inverter. So that can add complexity.

In the washup, a Victron dealer should be consulted to nail all this down for you in practice. But it's good to have some background before then..

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