Maximum Charging Amps

RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
Can anyone tell me what the maximum charging amps are for the Trojan L16P-AC (420 ah 6v). I have 8 of them for a 840ah 24v bank. The reason I ask is that today I turned on my generator to assist in charging my bank after some heave overnight use, and together with the solar panels, I was putting about 120 amps into the bank (80amp on the inverter/charger and 40 amp from the solar charge controller). Unfortunately I only had a 110amp fuse protecting on the battery bank and it blew. I got a bigger fuse (200amp) but wanted to double check that I was not putting too much current into the bank to begin with. Its a 3,500 watt inverter, so I know that I may be able to technically pull ~145 amps out of it, but net even sure that is a good thing for the size of the bank (I usually limit it to ~40 amps before the generator kicks in).

Any good numbers I should be looking for? Any general rules to go by?



  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Maximum Charging Amps

    Good thing you had that 110 Amp fuse, as that is about the limit for 840 Amp hours of battery: 109.2 Amps being 13% (the recommended max for FLA's).

    Don't keep feeding them such high current unless you own stock in a battery company.
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Maximum Charging Amps

    I think it was the "perfect storm" of charging. Nice sunny day right at peak production, and the bank low enough that it was still in bulk charge when the generator started. So, that brings up something that I need to figure out. How to prevent it in the future. My generator is set to auto start if there is more than 1kw pulled from the battery bank for more than 60 seconds. So lets say its a nice hot day (like it is today) and I need to run the A/C. That will cause the generator to kick in. And if the charge controller is putting in its full 40 amps, then I will have an issue.

    Also, the generator is set to autostart at 75% SOC ... so if I am pulling more power from the bank that I can replace with the panels, and the SOC gets low enough, the generator will start and blow the fuse.

    I guess my only option is to limit the charing output from the inverter/charger (Outback VFX3524) from 20 amps AC to about 16 or 17. That way I won't ever put more in from the generator than the system can handle (if the CC is running full). But it seems like a waste of possible power (~500 watts) that the generator can give me.

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Maximum Charging Amps

    Kind of have to wonder what you've got for an array. 840 Amp hours @ 24 Volts would want around 2600 Watts. That should give a 10% charge rate from the panels (84 Amps). Sounds like you've got about half that much.

    I'd put a 150 Amp breaker on the batteries so you can reset it if needed rather than have to replace the fuse if this happens again. Otherwise reprogram the inverter to limit AC charging to 8 Amps (roughly 40 on the DC side; Outback uses AC Amps to set the DC charging - confusing!) You might want to lower the SOC for the auto gen start too, as 25% discharge is usually normal battery cycling and who wants the gen starting up all the time?

    When the gen does start all loads shift to it and no longer become a concern for the inverter & batteries. About 1kW for the charger at full 40 Amps DC (can't remember the exact PF) and another 1kW for the loads adds up to a fully-loaded 2kW generator if needs demand. So how big is the generator?
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Maximum Charging Amps

    I've been down this path before. :)

    Lets do some simple math. I pull (currently) about 160 watts of power (24/7). That makes it about 3840 watts per day (its a bit more depending on temperature and such). To have 3 days worth of reserve power, I need 11520 watts. An 840 amp battery bank @ 24v yields 20160 watts. With a 50% DOD, this leave me with 10080 watts. Not enough reserve for 3 days. So my batter bank is not actually large enough, but its close (and if needed, I can remotely shut things down to extend the power if required).

    Now, lets do the math on the array. Pulling 160 watts over night (lets say 14 hours) means I am using 2240 watts. The larger the array, the faster it will charge the bank back up. With my array of 6 - 210 watt panels, that means I am done with my bulk charge about 11am .. before it even gets to max output of ~43 amps. I usually go form bulk to float around 38 amps. By adding more panels all I end up doing is charing up the bank faster, but not really getting the full 10% charge rate. To actually get the 10% rate, I would need to use MORE power overnight and add more panels. But by using more panels, I now have a bank too small for a 3 day reserve .. so I would need to add more batteries, which means more panels, which means I need to use more power. Its a never ending circle. :)

    I have talked to a lot of people that have this exact same issue. As it stands now, I go down to about 90% SOC overnight, so the batteries are never really discharged that much (85% of the year this is what happens). Sometimes when I am onsite and fully utilize the system, I get down to 78% overnight. During the winter the array can recover just fine from that in a single day. But during the summer I will usually be running the A/C so that panels can't keep up.

    The generator was added as a backup for the 3 day reserve and to allow me to use the A/C during the summer months. Its a 6kw Generac EcoGen LP. Very quiet and (so far) very reliable. 6kw is a bit larger than I needed, but It fits right in with the inverter (half load on the generator is max output of the inverter, plus I can still hook up an RV if guests come out). I choose 75% as my generator SOC kick-in as a compromise of not letting the bank DOD get too low to help maintain the batteries, and not letting the generator have to run too long to get everything back up to full charge (actually the generator only runs to about 95% SOC .. its a waste of LP to let it do the absorbe when the panels can do that). I could actually reduce the kick-in to to say 65% SOC, and may do that if I find the 75% figure too high .. but right now, its just about right.

    I also usually do an equalize charge once a month when the system does a exercise test.

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Maximum Charging Amps

    Only my opinion of course, but it sounds like you've fallen into the "Three Day Reserve" trap.
    Okay, let's do the math again.
    Your average draw is 160 Watts for 24 hours: that's 3840 Watt hours.
    Your 840 Amp hours @ 25% DOD should provide: 210 Amp hours or 5040 Watt hours DC; about 4.5 kW hours AC. More than enough to meet demand.
    Day Two is the second 25% of the battery's capacity, bringing it down to the maximum 50% DOD.
    Day Three you start the generator.
    The 10% charge rate is not a constant current flow or mandatory charge rate: it is a potential peak current that is a short-cut to sizing the array properly. Your array should be capable of 10% peak charge current rate, whether it needs to provide it or not. That means 84 Amps @ 24 Volts or 2016 Watts, less typical 77% efficiency = 2618 Watt array. This could be tailored down to 80 Amps so that it could all run through an FM80: about 2500 Watt array.
    Using the Icarus formula and a minimal 4 hours of sun that array should provide 5 kW hours of AC power per day; more than enough.

    Again, only my opinion. But it does sound like you've actually got more battery capacity than you need (using only 10% capacity overnight) and definitely too little panel for the battery you've got meaning you're running the gen more than you should have to. I won't say the gen is too large as I understand the need for it to supply A/C as well, and that can be a big power user.

    As always, the battery bank should be sized to provide the daily power needs and the array sized to recharge the battery bank. Statements like "I usually go form bulk to float around 38 amps" are troubling. You should not go from Bulk to Float as that implies it skips Absorb altogether. And never mind the current, as the transition between charge stages should be determined by Voltage levels and time (as in Absorb time; Absorb End Amps is not reliable). Also "But by using more panels, I now have a bank too small for a 3 day" does not make sense. Refer to the first sentence in this paragraph.

    Not to be mean, but you seem to be getting hung up on using the wrong numbers in the wrong way. I'm sorry to say it's a very common mistake too.

    We all just want your system to work the best it can. :D
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Maximum Charging Amps

    So, basically what your saying is that I'm probably sized right for the load and the batteries, just not for the panels. And to be sized right for the panels (as it stands today), I would need about 8 more. That would basically be around $4500 .. the cost of about 16 of these batteries. So my best guess is that by not hitting the 10% charge level, I'm degrading my bank by a year or so. If I can get 4 years out of these 5 year batteries, then thats a pretty good financial trade-off.

    However, I understand your figures. It all makes sense. I do plan on adding a 1kw wind generator into the mix this year, so that should help offset the charging difference.

    As for going from bulk to float .. I was in error.. I meant from bulk to absorb. And as per Outback support, I have my absorb time set to a max of 5 hours and a return amps of 5.4 amps (with all the recommended voltages set and temp compensated).. That usually gets me right at the correct return amps in 3.5 to 4 hours. Not sure how that works out with the correct charging and all, but that is what I was informed to set it at.

    I'm sure everything can be optimized better by someone like yourself if you were here adjusting the system, but I can only do whats recommended and what makes the best financial sense while still meeting my needs. Is it optimal? No. Will it degrade my system? Probably some. It it going to cause my system to stop running? No.

    But, that still does not solve my original issue... How to configure my charger so it does not overcharge the batteries if the array is also charging at its max potential. Adding more panels would just make it worse. I would never be able to run my generator to actually charge my batteries .. well, not unless I ran it for 10 hours. This seems like something that Outback's systems should be smart enough to figure out (priority from the charge controller before the inverter/charger), and not go over a set charge rate.

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Maximum Charging Amps

    Whoa! $562 for a 210 Watt panel? You live in Canada like me? :p NAWS lists them at $355:

    One of the advantages (or problems) of larger arrays is the "extra" power available during daylight hours when the batteries are fully charged. If you can harvest this "surplus" through load-shifting you might be able to run your A/C for "free".

    I don't like to use Absorb End Amps due to the vagaries of load draws, but about 4 to 5 Amps would be right for your battery bank so you're probably good at 5.4 Amps.

    Don't put money into wind power until you do a good site analysis and see if you've got the kind of sustained winds necessary to make it worthwhile. Turbines need to be in "clear" air with speeds of 20 mph or more and no gusting to operate best. Any brand claiming otherwise is probably lying.

    The only way to definitively prevent a repeat performance is to reprogram the VFX to limit AC charging Amps to 8, which is about 40 DC. Unfortunately there is no easy, automatic system that will check the incoming current from both sources and keep the combined total below the battery bank's desired maximum.

    But I'm going to have a look in my big book of Outback and see if communication through the HUB or adding one of the Flexnet options might change that.
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Maximum Charging Amps

    They have come down about $50 a panel since I last bought them, but even at that price, once you add in the fact that I need to get another charge controller, new mounting (pole mounting now, I've run out of room on the deck) and the necessary connectors and cables, its probably more than $4500. But that was a good number to start at.

    Yes, with a larger array I could run the A/C off of it. And have enen thought about it. The A/C unit that I use runas about 900-1200 watt (depending on temp and compressor loads). So I can almost run it off the array now (well, I can right at noon, but then I'm not charing the bank). Two more panels and I could do it .. but its not worth it for just 10-15 days a year (i'm only here during the weekends when there is no moon out).

    I'm still "deciding" on the whole wind thing. I've collected the weather data for this site for almost 3 years now, and its not anything special, but there are times when it would be a good alternative to the solar (if the sun is not shining, its a good bet that the wind is blowing around 10-20 mph). I have also thought of just getting some solar trackers for the panels to get some more use out of them. Unfortunately, I'm close to maxed out on my FM60 charge controller, so if I do anything else, I need to get another one. Right now I have a 3x2 array (3x the voltage, 2x the amps). Putting two more panels would get me a 4x2 array.. and that would then max me out on the output amps.

    I'll see about lowering my charging amps. But I don't think i will need to go as low as you suggest. With my current array maxing out at 43 amps, I just need to make sure that I dont go over about 67 amps DC. That would keep me under the 110 amp limit (13%). If I was to add more panels, I would need to adjust accordingly.

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Maximum Charging Amps
    How to configure my charger so it does not overcharge the batteries if the array is also charging at its max potential.
    This seems like something that Outback's systems should be smart enough to figure out (priority from the charge controller before the inverter/charger), and not go over a set charge rate.

    I have the same problem... sometimes I need to run the generator (for power tools) on a sunny day.
    This issue has been discussed recently:

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • silvertopsilvertop Solar Expert Posts: 155 ✭✭✭✭✭
    Re: Maximum Charging Amps

    Be very carefull above 13% chargeing rate If you ever tried to charge a bad car battery It will try to charge the max in to the battery,and will not stop and can cause what is called thermal runaway. This can also happen with a very high Amp rate of charge
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Maximum Charging Amps

    that is quite a bit of current you're pumping into your batteries, but shouldn't be bad for them. we generally recommend for trojans about 10% and the range we recommend for batteries in general is between 5% and 13%. now for an 840ah battery bank at a 120a charge rate is 14.283% and is only a tad over the 13% area we recommend. it won't hurt them at that rate and will probably result in a tad more maintenance than normal. just be sure your batteries are not heating up badly and if all looks fairly good then you should be able to continue to do it.

    to cover my butt and yours i would recommend contacting trojan and ask them what a top end charge might be for their l16s and what might be considered overheating the batteries. if you do this let us know what they say.
  • 2manytoyz2manytoyz Solar Expert Posts: 373 ✭✭✭
    Re: Maximum Charging Amps

    I think in actual use, you'll find you can't supply that much current for long, before the battery voltage starts coming up, and the charge rate starts to taper off.

    In a weekend long test I've done running critical loads from the 900AH battery bank exclusively, I used solar power, plus a 75A Iota charger to replenish the battery bank. IIRC, the battery bank was ~ 75% capacity remaining.

    The solar array can supply (measured) almost 50A. The Iota (measured) can supply a tad over 75A. Combined, 125A. But while monitoring the current going into the battery bank, the charge current quickly dropped off within minutes after starting the charging process. By applying a heavy load (1500W heatgun), I could get the charge current to spike for a few minutes. So the charge capacity was available, but the increased voltage caused the charge rate to decrease.

    Solar charging:

    Attachment not found.

    Iota charger:

    Attachment not found.

    After a few minutes of charging:

    You can see the battery voltage is 13.3V (and climbing), and the charge current has already tapered off to 100A:

    Attachment not found.

    After a few more minutes, the charge rate dropped to 75A, then dropped off even more within an hour.
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