Battery questions

This is a series of questions for all the experts about a power problem that needs solving:

My colleagues and I are working on an off-grid sustainable construction project. We are not going to be near any source of electricity. For budgetary and logistics reasons, solar panels and generators are not an option. What we do have access to is industrial/forklift batteries. We need to be able to power several tools and possibly some small appliances. Our estimated power use is 5kWh a day. The plan is to charge batteries with this amount of power every day at a project management office in a town near the site. The project will only last from several months to a year, so a particular brand or whether the batteries are new/used is of little concern. We have done just enough research to realize we don’t know enough to know where to begin solving this on our own.

That being said, here are our questions:
  • Is there a preferred type of battery that works well for being used 12-18 hours at a time? Deep cycle? Golf cart? Forklift?
  • For power tools and lighting equipment, is there a particular voltage the battery needs to be? 12V, 24V, 48V?
  • Our project office will likely be in an industrial park. When looking for space, what electrical specs should the building have?
  • Can most of these large batteries recharge within a 24 hour period?
  • Weight: we will have to haul these batteries to the site daily after charging. Which batteries are light enough to reasonable pack up every day and carry to the site (50 lbs or less)?
  • Wiring: for sustained use, should we wire the batteries as series or parallel?
  • Safety: what are the real dangers of handling the batteries and what precautions can we take to protect ourselves?
  • Does the charger have to be the exact same voltage and Amp-hours (Ah)?
  • What type of inverter do we need? 5kW? Does the type of inverter we use depend on the type of batteries?

I thank you in advance if you are able to provide any answers or offer advice


  • CALLDCALLD Solar Expert Posts: 230 ✭✭
    My advice is firstly to make sure you have ample battery capacity. For a 5kwh per day use, multiply that by 2.5 to work out the required battery capacity to ensure a good reserve margin. The reserve margin is needed to ensure you don't discharge the batteries beyond their sustainable level. 50% is widely considered to be the safe depth of discharge for any deep cycle battery. The reason I used 2.5 as a factor instead of 2 is because many batteries don't live up to their claimed capacity. So in your case you want a battery capacity of 12.5kwh. The voltage should be 48v. The Ah capacity should be at least 300Ah. The total battery weight would be around 500kg or 1200 pounds. Assuming this battery is made up of 8x6v fork truck batteries the weight of each is unlikely to be easily manageable by workers. I would thus recommend you keep the whole setup in a van or other suitable vehicle and just hook-up wherever needed for charging or discharging. The size of the inverter will depend on the maximum load which will need to be run - multiply it by 2 for a good safety margin.

    Battery health will need to be monitored so get a good hydrometer and a few bottles of distiller water for topping up in needs be.

    Generally recharging takes 8 - 16 hours with a good charger. Your charger will need to be able to deliver at least 30amps at 60volts.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,184 ✭✭✭✭✭
    Why would you not just use a generator like most of the world does for this kind of project. Every remote home I have been involved with does not do what you are proposing when building the structure, it sound like that is what you are doing? If you need 5 KWH you cannot expect to charge the battery with only 5KWH. There is loss in all battery charging currently on this planet.
    Good Luck and hope you are good at lifting!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 30,514 admin
    Just to work some basic numbers:

    5,000 WH * 1/0.85 inverter eff * 1/0.80 maximum daily discharge (for fork lift battery) * 1/24 volts = 306 AH @ 24 volts

    Get the next size of battery (allow for aging, cold weather, just in case):

    Crown Industrial Battery - 24 Volts, 530 Amp-hours ($3,000)
    • 24 Volts
    • 530 Amp-hours at the 20 hour rate
    • Dimensions: 26.13"L x 12.69"W x 23.25"H
    • Weight: 930 pounds
    Warranty: 1500 cycles to 80% DOD for five years full replacement.
    Note: These batteries are very heavy and we highly recommend that you have a small crane or forklift available to move and load/unload it.

    The forklift battery would need around a 53 amp charger (minimum) @ 24 volts and at least 8 hour a night to recharge.

    Or get around 8x series * 3-4 parallel strings of these (24 to 32 batteries) or so LiFePO4:
    • 3.5 kG * 32 cells * 2.2 lbs/kG = 246 lbs (plus hardware)
    I don't know if the price of $135 per cell is real/high/low... But 32 of them would cost ~$4,320 if the cost is correct.

    For the Inverter, you don't (necessarly) need a 5 kWatt inverter for a 5kWH system... If you are averaging 4 hours at/near full power, then the average peak power would be:
    • 5kWH/4 hours per day = 1.2 kWatt inverter "nominal" output.
    Or, for a $1,000 you can get a 50 lb Honda eu2000i + 2 gallons of gasoline (another ~12 lbs+container) per day (1,600 watt genset, pretty efficient down to ~400 watt average load, and pretty quiet/reliable) ($5-$10 per day fuel costs + quart of oil per week).

    Get a second (cheap/noisy) 3.5-5 kW genset for powering large tools, pumps, etc., when needed.

    That is the drawback of batteries... Lead acid batteries--1,000 lbs of lead vs 12 lbs of gasoline. The lithium batteries are much better, but still not cheap/light.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Alaska ManAlaska Man Solar Expert Posts: 252 ✭✭
    Move your grow-op............
Sign In or Register to comment.