What is in a cheap "PWM" solar battery charger/controller? - Page 1
Forum section: electronics / beginners

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2022, 06:14:39 am
content: For a bit of fun: I'm thinking of getting a cheap second-hand roof solar panel and combining it with an ~80Wh tool battery to try and power some small projects.These really cheap "PWM" (not MPPT tracking) battery charge controllerslook like they are configured for charging lead acids, not arbitrary numbers of Li-ion or LiFePO4 cells in series.Question:What power control method do these cheap "PWM" solar charge controllers use? Random thoughts:1. They directly PWM the ~40V into your lead acid battery (unfiltered high voltage spikes! maybe lead acid can survive this?)2. They're really just buck switchmode power supplies (filtered) with fancy screens.3. They do something weird like short the entire solar panel out each PWM cycle (dissipating power in the panel & transiently lowering the voltage)Teardown videos I have seen show no notable inductors inside! That combined with the "PWM" advertising terminology makes me think they doing something magic.

author: gf
author_rank: Super Contributor
timestamp: November 03, 2022, 07:28:33 am
content: PWM directly into the battery. But why do you think there were "high voltage spikes"? A solar cell is rather acurrentsource, so when you connect it directly to a battery, then the battery determines the voltage and the solar panel determines the charge current. PWM can then control/limit the (average) charge current to prevent over-charging. The no-load voltage of the panel must be higher than the battery's charge voltage, and ideally, the MPP voltage of the panel should be approximately the same as the battery's charge voltage. If the MPP voltage is higer than the battery voltage, a PWM charger cannot utilize the maximum power of the panel. For instance, if the MPP of your panel is at (say) 36V and you charge a 12V battery, then you can only utilize ~1/3 of the maximum power the panel could deliver.Regarding (3): Neither when the panel is open, nor when it is shorted, you can harvest any power from the panel. In both cases, the sun simply heats up the panel. If you consume electrical power from the panel, then it does not heat up so much.Edit: Regarding (2): Most MPPT charge controllers are switch mode buck converters (some can also boost). Consequently, if the MPP voltage is higher than then battery voltage, then the charge current can be higher the short circuit current of the panel. OTOH, a PWM charger can never charge with a higher current than the short circuit current of the panel.

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2022, 10:23:10 am
content: That makes much more sense, thankyou. Yes I was (incorrectly) thinking of the panel as a voltage source.I'll have to read into Li-ion and LiFePO4 charging characteristics to see if high C current pulses are permissible.

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2022, 10:28:21 am
content: Quote from: gf on November 03, 2022, 07:28:33 amTook me a few reads to make sense of that. The charge current is the output of the buck converter, not the current from the panel. Yes that makes sense