# Time to recharge?

Solar Expert Posts: 56 ✭✭
If you had 12 Trojan L16RE-2V for 24v total, and they were discharged 50 percent, how long would it take a 12.5 kw generator to recharge them (recharge point of your choice, ambient temperature of your choice, other assumptions of your choice) for the next discharge cycle?

• Solar Expert Posts: 5,183 ✭✭✭✭
What are you going to use to charge them with?

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Solar Expert Posts: 3,009 ✭✭✭✭
Does your generator have a 30 volt DC output for charging batteries? If so, what is the available amperage?
If not, then you will need a battery charger powered by your geny. If you already have that, what is it's amperage output? What is it capable of? Does it's output drop off as battery voltage increases?
lasitter wrote: »
If you had 12 Trojan L16RE-2V for 24v total, and they were discharged 50 percent, how long would it take a 12.5 kw generator to recharge them (recharge point of your choice, ambient temperature of your choice, other assumptions of your choice) for the next discharge cycle?

• 12,500 Watt genset * 0.80 derating * 0.80 typical charger eff * 1/29 volts charging = 276 Amps estimated maximum charging current
• 12,500 Watt genset * 0.50 "minimum optimum load" * 0.80 typical charger eff * 1/29 volts charging = 172 Amps estimated minimum "optimum" charging current
Trojan L 16RE-2V are around 1,110 AH @ 20 hour rate. We recommend charging at 5% to 13% rate--For an AC charger you could possibly go as high as 20%-25% if you monitor temperature (check with battery vendor/Trojan to be sure).

For sake of discussion, lets say 13% rate of charge--About the highest charge rate for batteries that are not temperature monitored by charge controller (typical, not sure Trojan specifically).
• 1,110 AH * 0.13 rate of charge = 144 amps rate of charge (less than ~276 amps available from your 12.5 kW genset
You may want to think about going to >= 172 Amps charging (50% of genset capacity) for a bit better fuel consumption (lightly loaded gensets tend to be less fuel efficient)

More or less, with reasonable charging current, the battery bank will accept maximum charging current from "zero" to ~80% state of charge (current regulation--Bulk charging):
• 1,110 AH * 0.50 = 555 AH
• 1,110 AH * 0.80 = 888 AH
• (888-555) AH / 144 amps charging = 2.3 hours
Next, from 80% to ~95% state of charge will be gradully declining charging current (fixed voltage regulation--Absorb charging). For a shallow discharged battery, perhaps ~2 hours of absorb. For deeply discharged battery around ~4-6 hours of charging:
• 2.3 hours Bulk + 4 hours Absorb = 6.4 hours
• 2.3 hours Bulk + 6 hours Absorb = 8.3 hours
Note that Absorb would be terminated when you reach around 2% to 1% charging current (~22.2 amps to 11.1 amps) and then fall back to float charging/turn off genset.

If you have solar, typically you would charge with the genset to 80-85% state of charge (say start the genset at ~7-8AM) and let solar carry the rest of the charging (save fuel, wear and tear, less noise). And/or as battery charging current falls, run other loads (well/irrigation pump, vacuuming, etc.) as generator has "extra headroom" for supporting other loads.

My first set of guesses...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 56 ✭✭
I like the durability and efficiency of diesel in general, and since I have 660 gallons of home heating oil capacity available, I was thinking something like this would do the trick: