Battery charger questions...

Re: Battery charger questions...

Most of the battery chargers are set for rated power/current... I would suggest that you purchase the charger and genset as a "matching set"...

There are some "solar RE" chargers that can be programmed for input current/output current/charge profile--but many of those seem to be in the inverter/charger category.

I have seen programmable fork lift charge controllers--but I cannot recall seeing a "off the shelf" AC to DC charger around here that is programmable to that degree. It would be nice to find one.

For example, your 800 AH 12 volt battery bank. Recommended rule of thumb charging range is ~5%-13%. Assume charger is 80% efficient. For various reasons, aiming at 50% generator capacity is another good rule of thumb.

• 800AH * 0.05 = 40 amps minimum recommended charge
• 800AH * 0.13 = 104 amps maximum charge rate rule of thumb
• 40 amps * 14.4 volts charging * 1/0.80 charge eff * 1/0.50 gen loading = 1,440 watts minimum generator (with minimum charge)
• 104 amps * 14.4 volts charging * 1/0.80 charge eff * 1/0.50 gen loading = 3,744 watts maximum generator (with maximum charge)

At this point, purchasing one or two more Iota Chargers (mix and match current ratings?) to run in parallel to charge your battery bank would be one way to get the loading higher on your genset (obviously, not cheap).

Another is to purchase a smaller genset (believe it or not, I do not get one cent from this recommendation, no matter how many times I say it--a Honda eu2000i 1,600 watt generator--if you can use pull start and it fits your needs otherwise)...

To compare running the two gensets charging your battery bank from 50% to 85% capacity. Assume ~5.5 kWhrs per 1 gallon of fuel for gensets. The Briggs will pull 50% fuel flow from 0-50% load (roughly) and the Honda will pull 25% fuel flow for 0-25% load--when set to ECO throttle). Assume battery bank and chargers are both 80% efficient. Also assume that charge controller outputs nearly rated current to 80-85% state of charge--past that point, you should either use solar array to finish the charge, or assume that you will run the generator for another 2-4 hours to get near 100% battery state of charge (pick 3 hours for exercise below):

• 800 AH * 12 volts * (0.85-0.50) = 3,360 WH = 3.36 kWHrs
• 40 amps charger * 14.4 volts * 1/0.80 eff = 720 watts from genset

Note that 720 watts is > 25% rated power (1,600 watt * 25% = 400 watts) of Honda eu2000i. But this is way under 50% of 5,500 watt Briggs (2,750 watts).

For charging the first stage (bulk):

• 3.36 kWHrs battery / 5.5 kWhrs per gallon = 0.6 gallons for Honda
• 3,360 Watt*Hours / (14.4 volts * 40 amps) = 5.8 hours run time

For the Briggs--assume 50% fuel flow if load is below 50% rated power.

• 5.5 kW * 1/5.5 kWH per gallon * 0.50 fuel flow * 5.8 Hours run time = 2.75 gallons

If you want to charge the final 15% of the battery capacity, then 3 more hours of run time--assume below 25% load on Honda, below 50% load on Briggs:

• 0.4 kWatts * 3 hours * 1/5.5 kWhrs per gallon = 0.22 gallons for Honda
• 2.75 kWatts * 3 hours * 1/5.5 kWhrs per gallon = 1.5 gallons for Briggs

The above is a series of SWAGs (scientific wild a$$ guesses)--but gives you a good idea of what the difference can be if you better match the battery charger and generator capacity...

So, for charging the same 800AH 12 volt bank with a 40 amp charger from 50% to 100% capacity is less than 1 gallon for a 1,600 watt genset and possibly over 7 gallons for a 5,500 watt genset...

My suggestion is to measure battery state of charge and how much fuel (and time) you are taking to recharge your battery bank with the current setup. You can even use a Kill-a-Watt meter to measure the output loads (watts and kWhrs) on your genset and compare against fuel flow. It will pretty quickly give you a good handle on the kWhr per gallon efficiency of your setup.

I would be interested to hear the results of your measurements...

-Bill