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My battery-powered wireless device draws about 50mA @ 5V. I’m thinking of powering this off a 12V 7Ah SLA battery that is topped up with a small solar panel (12V, maybe 500mA or so). 12V to 5V will happen with a DC-to-DC converter. Can I just connect a solar panel (with diode) straight onto my battery or is this where I need a charge controller? If the latter, what is the reasoning behind it?

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  • \$\begingroup\$ You can connect small solar panels to large batteries without a controller, because the panel current is not high enough to damage the batteries. In your case, I would not advise it, because you are talking about a 0.5A current and a 7Ah battery. This has the potential to damage the battery. You could use a very simple and low cost charge controller which does nothing more than disconnect the panel when battery voltage gets too high. \$\endgroup\$ – mkeith Feb 4 '16 at 7:12
  • \$\begingroup\$ @mkeith I think SLA batteries are fairly forgiving regarding their charge method so I might take your hint and use my micro to simply probe the voltage and do the disconnect using a fet. \$\endgroup\$ – captcha Feb 4 '16 at 22:06
  • \$\begingroup\$ That could work OK. One thing, though. You need to add some hysteresis. In other words, if VBAT > 13.5 disconnect. But once disconnected, do not re-connect until VBAT < 12.5. Those numbers are somewhat made-up. You will have to research your battery type to determine safe upper limit, and use judgement or experiment to determine the lower limit. But without hysteresis, the FET may switch on and off rapidly which would not be good. \$\endgroup\$ – mkeith Feb 4 '16 at 22:21
  • \$\begingroup\$ @mkeith Understood. That makes sense. I've also noticed that not every SLA battery I have seems to have the same fully-charged resting voltage, so I could even tweak it a bit more depending on the battery I use. \$\endgroup\$ – captcha Feb 4 '16 at 23:34
  • \$\begingroup\$ I don't know any exact SLA numbers off-hand, but I think the alternator in my car goes up to 14.2V in normal operation. \$\endgroup\$ – Robherc KV5ROB Feb 8 '16 at 4:53
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It's recommendable to use a charge controller between the solar panel and the battery. The charge controller is there to avoid damaging your battery by overcharging. When a lead-acid battery gets charged above about 98%, oxygen and hydrogen bubbles form around the internal plates in the battery.

In a "maintenance-type" battery, this vents out and can be replaced, in an "SLA" battery it can cause irreperable "gassing," but in "AGM" and "gel"/"deep cycle" batteries, the bubbles stay tralped against the battery plates. These bubbles reduce the available current & capacity, while increasing the internal resistance of the battery.

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  • \$\begingroup\$ I've got a commercial battery charger and will study its charge characteristics on the battery I intend to use. I'll probably roll my own and simply disconnect the pv when the battery voltage exceeds 13.5V as per the suggestion by mkeith. \$\endgroup\$ – captcha Feb 8 '16 at 0:17
  • \$\begingroup\$ Research the best numbers to use for your battery type and temperature. 13.5 might be a good ballpark number, but I kind of pulled it from memory. Battery temperature matters. \$\endgroup\$ – mkeith Feb 8 '16 at 4:44

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