GC2 vs. grp 27DC 100A load test

I have 3 deep cycle battery set ups that I load tested at 100A for up to 75 seconds. Based on the results, which set up is best suited to driving a 100A (1200 watt AC sump pump on inverter) load that draws 29A ac on start up? the marine batteries are identical - but one appears to have more capacity than the other.

1. Two 6V GC2 golf cart batteries rated at 225AH connected in series for 12V:
No load charger off: 13.45V
15 seconds (at 100A): 11.78V
30 sec: 11.78V
45 sec: 11.78V
50 sec: 11.78V
75 sec: 11.78V
Load off: 12.42V
15 sec rest: 12.5V
30 sec rest: 12.53V

2. One group 27DC marine DC battery, 115AH:
No load chgr off: 12.91V
15 sec (at 100A): 11.92V
30 sec: 11.91V
45 sec: 11.91V
60 sec: 11.89V
75 sec. 11.89V
Load off: 12.55V
15 sec: 12.61V
30 sec: 12.64V

3. One 27DC marine battery, 115AH
No Load chgr off: 13.53
15 sec (at 100A): 11.65
30 sec: 11.68
45 sec: 11.69
60 sec: 11.70
75 sec: 11.71
Load Off: 12.40
15 sec: 12.47
30 sec: 12.49

would running both 27DC batteries in parallel be better than the 2 golf cart batteries?

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: GC2 vs. grp 27DC 100A load test

    it would be pretty much a draw as far as capacity goes as the golf cart batteries will be about 225ah at 12v and the paralleled group 27s will be about 230ah. many here are of the opinion that the seriesed batteries are more desirable due to imbalances paralleled batteries can see. imo, go with the golf cart batteries.

    now 1 thing that bothers me is you say this pump which is ac is drawing 29a. i hope this is not 29a at 120vac as that is more than either battery bank can handle. if that is 29a dc then it is feasible, but be advised that at 29a for 12vdc that that is at an 11.6% rate of power consumption and in under 5hrs you will reach 50% dod. you don't want to drain your batteries below 50% dod.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: GC2 vs. grp 27DC 100A load test

    The question you have to ask yourself becomes more about the ROI. The GC2's will last about 5-6 years with minimal care and the Marine Batteries will last 2 years if your lucky with that kind of draw.
  • System2
    System2 Posts: 6,290 admin
    Re: GC2 vs. grp 27DC 100A load test

    thanks for the replies.

    the 29A is at 120Vac but is only for a brieft moment (<1 second) while the pump is starting. After the start up surge the pump will draw about 9A at 120Vac. and then it will only be on for 10 seconds or so before it shuts off (while the sumps fills with water again).

    that means the batteries will have to deliver about 330A for a short burst at start up. I've load tested all 3 at 350A and they stay well above 10.5V but the voltage does begin to drop off rather quickly. at 350A the GC pack seems to drop off quicker than a single 115AH 27 marine battery - perhaps because the marine batteries are dual purpose??
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: GC2 vs. grp 27DC 100A load test

    I think the term " Marine " is a loose standard. To some it means they have a screw post that will accept ring terminals. To others it is a dual purpose battery. How much different is something you have to dig out. I have a couple Optima batteries, while they have blue top, red top and yellow top, they never really admit that any are true deep cycle.

    The GC2's will accept a much deeper discharge and a faster charge back rate. @ 9 amps AC your pulling 80-90 amps DC. At that amp pull you need 2/0 cables, I do over kill and use 4/0, but thats me.

    I have had sets of GC2's go 8-9 years with no issues, it depends on how many cycles and how they are treated.
  • jagec
    jagec Solar Expert Posts: 157 ✭✭
    Re: GC2 vs. grp 27DC 100A load test

    So that's, what, 290 amps DC startup, 100 amps DC running? For only about 10 seconds at a time? How often does the sump pump run?

    To me that sounds like exactly the sort of use that starting batteries see. Since those marine batteries are dual-purpose, I'd almost think that they'd do a better job of delivering that kind of juice reliably than the true deep-cycle batteries.
    However, another thought:

    I recently added a battery back-up sump pump system to my house. My main sump pump is one of those typical large AC units. I haven't measured its power requirements, but I'm sure that they are very similar to your pump. But for my backup system, I picked up a small, 500 GPH 12 VDC automatic bilge pump, and installed it a couple of inches above the main pump.

    I have found that water never really comes RUSHING into my sump, even in a thunderstorm. The worst is a steady trickle. Thus, the smaller pump is more than capable of handling the inflow if the main pump fails or the power goes out, and it's a heck of a lot cheaper, more efficient, and easier on the batteries than trying to use an inverter to power a 1/3 hp AC monster. Quieter, too; I'm almost tempted to unplug the AC pump entirely.
  • System2
    System2 Posts: 6,290 admin
    Re: GC2 vs. grp 27DC 100A load test

    well that's the thing - I get a lot of water in the sump during heavy rains. earlier this year we got 7 inches of rain in a little over an hour (chicago burbs) and the drain tile was like an open fire hydrant in the sump. there's no way a DC battery back up pump could ever keep up with that. in fact, I had 3 pumps going at once - my main AC pump, the DC back up pump, and a multi purpose pool type pump with a hose running out the back window. all 3 pumps ran continuously for almost an hour before the water in the sump began to subside (got up to 1" below flood level). I had to pull the battery out of my car to run the backup pump because the stupid basement watchdog battery failed (that's another story).

    so in that case any brief power outage would cause a flood. hence my current attempt to build an emergency back up power source for the main AC-powered pump. and I plan to also replace the whimpy DC backup pump with a 2nd full power 1/2hp 5000 gph AC pump, also powered by a 2nd inverter/battery bank.

    that storm was an extreme case (record rainfall). during a typical heavy rainstorm the pump cycles about every 5 minutes or so, and is only on for 10 seconds or so while it drains the pit.

    all of this begs another question --- how is it possible to have a 2200 gph DC powered pump that only draws 10A at 12VDC (like the big basement watchdog pump), when a comparable AC pump draws 7A at 120VAC? that would imply DC powered pumps are 7-10 times more efficient than AC pumps -- I know that can't be true, so what am I missing?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,422 admin
    Re: GC2 vs. grp 27DC 100A load test

    A "good" DC pump may be 50% more efficient than a typical AC induction pump... But, from the most part, it appears the DC pumps are designed and rated to push against a much lower head (back pressure).

    But, without looking at specific pump curves--I would be just guessing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset