stephenrye avatar image
stephenrye asked

Blue Power IP67 Charger as DC UPS will it charge properly if battery under fluctuating load

Hi i basically want a DC 12v ups, for a sercurity system.

they do exist but thus far seem rare and expensive,

So I had the idea of using a battery Blue power ip67 charger to power the cameras/ led/ alarm system, while also charging a battery.

The charger is a 7 Amp, the systems draws between 1-5 amps, usually less then 2 amps.

Will the battery get properly charged and maintained, or will the fluctuating load mess up the charger's charging capability resulting in poor battery life span?

I think if the charger used volts to determine its status it would be ok but if it uses current than not so.

I plan to use one or two 7AH sealed lead acid battery, would it make any difference if i used lithium?

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Hi Stephenrye

What you are proposing is not much better than a boxed PSU with battery Backup.

Going with the best case scenario of your security kit drawing 1 amp with a single 7AH (Yuasa I would imagine) battery you would get 7 hours of supply in theory however the voltage would start to sag at about 5 hours of use and left connected your equipment would power down and leave you with a damaged SLA battery as it is never a good idea to discharge any lead acid battery beyond 50%.

With a load of 5A you would only really get about 1.5 hours of backup.

I would suggest that you purchase a reasonable size 200+ AH leisure battery and connect that to a decent charger like the Blue power. I have two of them myself and they are excellent chargers. If you are familiar with electronics I would use a 12.5V zener diode across the drain of a mosfet, put a diode across the mosfet output and connect that to a relay. That would then switch off the relay if the voltage falls below 12.5V (the lower limit of your battery capacity). Use the common and normally open connection of the relay to power a 12V DC to DC charger. These are used in caravans and campers to run any 12V equipment and will give you a nice steady 12V until the supply battery runs out. If you use a DPDT relay you could use the other set of contacts on the relay to signal that the system has shutdown. This depends on the type of communications device you use so make sure you choose one that has its own battery backup.

If you do not fancy tackling the electronics side you could go with a Victron battery monitor / state of charge meter and use the relay contacts on that to shut off the load or signal a power failure by setting the alarm voltage to something like 13V. There are a few ways to achieve your goal, it all depends on how much you want to spend. I have set a system like this up myself so would be happy to offer any further help if you need it.


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kai avatar image
kai answered ·

Its usual to have a load connected in parallel with the battery and charger - they're designed to work like that. "Maintenance" of cell chemistry is largely done through management of voltage over time (raise for absorption mode, drop for storage mode). As long you don't try to charge the battery at a higher C rate than recommended by the battery mfg, your scheme looks ok.

Lithium is more expensive (battery + potentially monitoring equipment). If weight isn't a concern I'd start with SLA.

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