I'm helping set up a system for a 3 year project measuring storm water runoff. This is my first project recharging batteries, any comments suggestions and questions will be very helpfull.

We have access to power from a 277 volt street lamp that turns on at night. We are planning to reduce power to 120 volts using a transformer, possibly this transformer, then connect a 3 phase charger possibly this one. will I need anything besides a transformer to do this?

When the power is turned on each night the charger will recharge a 12V AGM SLA deep cycle battery which will need to store enough power to run 3 Raspberry Pis for about 15 hours a day. The standard charger for a Raspberry Pi is 5.1v 2A. We are looking into switched mode power supplies to efficiently lower power to the 5V 2A our Pi's require. Any suggestions about switched mode power supplies? could I take one out of a wall charger and use that?

I have read that they draw between 700-1000mA depending on what they are doing, our Pi's wont will probably be drawing closer to 700mA. Could we safely get away with supplying less than 5v 2A? Will a 100aH battery be sufficient?

Thanks, Danny

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    \$\begingroup\$ Nothing about 3 phases is the links you posted, and you even don't need 3 phases for such small charger. Do you even understand what is three phase system? \$\endgroup\$ – Marko Buršič Oct 27 '15 at 15:46
  • \$\begingroup\$ Most streetlights run from a single phase line that is derived from one leg of a 3-phase supply. 277 Vac is one leg of a 480 Vac 3-phase feeder ("Y" connected). You would NOT normally have more than one phase available at the streetlight. \$\endgroup\$ – Dwayne Reid Oct 28 '15 at 22:24
  • \$\begingroup\$ It sounds as if you haven't yet characterized your power consumption. You really need to do that before doing any power and battery capacity calculations. In particular, you should spend some time figuring out how to reduce power consumption of your system. \$\endgroup\$ – Dwayne Reid Oct 28 '15 at 22:26

Make sure you over rate your battery capacity so that it cannot discharge to less than 60% of capacity after any expected power outage or have a under voltage cut out. Deep discharge is very destructive to consumer lead acid batteries.

Worst case figures:
Discharge margin 60%
Switching voltage regulator 80% efficiency
Low Battery voltage 10.2V (pick happy number)
Current required Max 6A (don't skimp here, it will not save money or much power)
Required 5.1V

Current required at 10.2V is 3A / 0.8 = 3.75 A Capacity for 15 hours is 56.25 Ah / 0.6 = 93.75 Ah

Your 100Ah battery should be up to the task if it is of a high quality and you do not cause it stress (high temp, deep discharge, over charge) or it will shorten life span.

You will need to charge at current high enough to fully replace over 60Ah in 9 hours of over 6.7A and the circuit consumption (worst case 3.75A) so at least 12A but better 15A at 13.8V because reaching final charge takes longer than a linear rating.

That would take about 2A from the 120V supply.

With unattended things a call out to check or replace a battery just once in 3 years may cost more than purchasing the next size bigger in all the components so this is what I would do and justify it just like that.

My calculations were fairly conservative except for the unknown recharge current required. I expect there will be tables and such to find out what peak current is needed to charge the batteries from 60% to 95% every night. Deep cycle are better and may be one of those over design ideas to implement, the 20% margin is what will save the day.
You may find that only 30% of the calculated capacity is used when everything is hooked up in the end (all the numbers were close to worst case). This will give you the safety margin for the reduction in battery capacity with age and the occasional unintended deep discharge due to roadworks, power failure, cable maintenance, human error.
Depending on the battery type you may have 10 to 100 deep discharge cycles before the battery is toast, try not to have any of these by design. It will be much cheaper to have a low voltage cut-out before deep discharge than having sporadic long power outages causing the batteries to degenerate too soon. Also do remember most all chemical cells perform worse when very hot (degeneration occurs) or very cold (capacity/stored charge vanishes).

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  • \$\begingroup\$ I read here that deep cycle AGM could be safely discharged up to 80%, is there a reason that's not correct? I'm confused about figuring out how long it will take to recahrge, could you direct me to a resource that will help me figur out recharging time? \$\endgroup\$ – fabricant Oct 28 '15 at 16:16
  • \$\begingroup\$ Also @Filek I am thinking about using this charger and 3 smaller batteries to power the Pi's individually. I figure that will make future maintenace and replacement less critical/expansive, and make it easier to find any issue with the system. \$\endgroup\$ – fabricant Oct 28 '15 at 16:22

You can get 3 or 4 amp USB car adapters. One of these should have no problem providing enough power for 3 Pis from your 12V battery for more than the time you need between recharging.

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Assuming that the calculations made by KalleMP are correct:

1) Be very careful on what deep cycle battery you purchase, 3 years service, assuming a discharge/charge cycle every day >1000 cycles. You will likely have to buy a larger capacity so that your percent depth of discharge is less so that you will get that many cycles out of a battery before it dies.

2) Recharging over 50 Ah over night when the lamps power turns on will be difficult. In general, lead acid charge slower than other batteries. If the battery is constantly undercharged it will die sooner. The battery manufacturer can help with that - a higher capacity battery can be Recharged at a higher amperage.

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  • \$\begingroup\$ The calculations were for discharge to 60% only which is conservative, wet cell lead acid batteries like that kind of service with regular recharge, in a well maintained car its battery can last for nearly 10 years and suffers much worse abuse. If the charge rate cannot be held high the maximum charge may not be full but if it reaches the same level of even 90% every day and never falls below 60% it will be reliable. If it gets to a lower charge each day then failure is certain. \$\endgroup\$ – KalleMP Oct 28 '15 at 22:14

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