How to charge bank w generator

vicariovicario Posts: 1Registered Users

My battery bank is down to 70%. I have an older Honda EG1500 generator that works well and I'm trying to figure out what type of battery charger to use it with. I have 6 Trojan 105s in parallel and not much sun in the forecast. The generator says 115V AC with max 13A. Honestly I don't really know what this all means so any help on what type of charger to buy and other tips is much appreciated. Thanks!


  • DRickeyDRickey Posts: 22Registered Users ✭✭
    Okay, quick primer: Watts = Volts x Amps. Memorize that formula, and its other common form (Watts / Volts = Amps). You need to calculate the charger size two ways: The largest charger your generator can supply, and the largest charger that is safe for your battery bank, then you use the smaller of those two.

    So your generator is telling you that it can produce 13 amps at 115 volts, 13 * 115 = 1495 watts. Chargers will usually be listed by their voltage and amperage, you know you have a maximum of 1500 watts, but you don't want to actually load it at more than 80%, so you have 1200W to work with.

    Divide that by the voltage of your battery bank, and that will tell you the largest size of charger you can use with that generator:

    1200 / 12v = 100
    1200 / 24v = 50
    1200 / 36v = 33 (chargers tend to come in increments of 5 or 10, either 30 or 35 would work)

    Now, your batteries probably aren't in complete parallel, because the Trojan 105 is a 6 volt battery and that isn't a normal voltage. Nor is it likely that it's 24v, because that would take either 4 or 8 batteries. So you have to be running at either 12v or 36v (probably 12v, because 36v is not a standard voltage for inverters).

    If it's 12v, then you have 3 pairs of batteries connected in series as pairs and in parallel for the sets. That will mean 33amps for each pair when charging at 100 amps, which is too high for the Trojan 105 (charge rate should be about 10% of the 20hr discharge capacity, for the 105 that is 10% of 225, or 22.5 amps). So working the other way, you get a maximum charging rate of 67.5 (22.5 x 3 =67.5). So you probably want a 12v charger in the 60-70 amp range.

    Keep in mind, this is all just 'rule of thumb', in actuality the voltage of a 12v system can be a bit higher or lower than the nominal, and being a bit off the 'ideal' value isn't a big deal. But you don't want to be way off, for example charging your 675 amp-hour battery set with 100 amps would boil off your electrolyte and potentially damage the batteries.
  • Raj174Raj174 Posts: 635Solar Expert ✭✭✭✭
    Hi vicerio,

    10% is ideal, but 5% is adequate. If you have trouble finding a higher amperage 12 volt charger or cost is an issue then 30 to 35 amps would be the minimum. Make sure it's a 3 stage charger. Bulk, absorb and float.
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • mike95490mike95490 Posts: 7,797Solar Expert ✭✭✭✭
    Another "gotcha" with chargers and generators, is the power factor (PF) As the PF lowers below 1.0, more out of phase I:V happens, and the generator with it's physical limits cannot supply enough power, and becomes overloaded.
     So to run a 1500w charger, you may well need a 3Kw generator.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • 2manytoyz2manytoyz Posts: 368Solar Expert ✭✭✭
    I have 8 golf cart batteries in my setup (T105 type).  I'm using a 75A Iota charger with a Yamaha EF2400iS generator, when the sun isn't cooperating.  This works well, but I was very surprised at the poor Power Factor of the charger!  Ideally, it would take 7.5A at 120VAC to make 75A at 12VDC.  Take a look at the pictures...

    That's 13.56 Amps at 120VAC to supply 74.4 Amps at 12VDC.  My generator is rated at 16.7 Amps, and I'm near the upper limit.  That said, it only stays this high for a few minutes typically.  The charge rate drops off as the battery bank voltage increases.

    I do like Iota chargers.  I'm using a DLS-55, with the IQ4 module, in my RV.  That might be a good choice for your battery bank & generator setup.
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