so I have 2 different pairs of 6 volt batteries on my toyhauler. one set from Costco, the golf cart bats, with a date on them of 4/12. they had been sitting in my garage for about a year-year and a half on a battery tender jr. , with it showing greenthe other pair came with the toyhauler, they are 2 6 volt agm batts, owner said they were still good and they seem like it but,,,,when I charge both pairs of batts to full charge, 13.5 or 13.6, as soon as I put a real load on them (like pulling 7 amps thru the xantrax 1800 inverter), the digital volt meter from the xantrax readout shows down to 11.1 volts, if I put a small load on them, like the heater fan, the volts fall from 13.5 to 12.5 or 12.4. from what I understand, going down to 12.3 volts, you have essentially used 40-50% of the battery capacity?my question is, are these batteries toast? obviously, each one of the pair of 6 volt batts should have at least 200 amp hours, so total 400 amp hours? and the volts drop pretty quickly, I would have assumed if I pulled 7 amps on the inverter, I should get at least 25 hours hours before I hit 12.5 volts (50% discharged). (200 amp hours divided by 8 amp load = 25 hoursany response is appreciated ( I am unsure of the date of manufacture of the agm's)Moved to Tech Issues forum from DIY.

If you have four 200 AH 6v batteries, your total capacity will be 400AH. Total usable or reasonable capacity would be around 200AH. It still doesn't explain the sudden voltage drop with using the Xantrex. If it's possible, I'd suggest to remove one pair of batteries from the circuit to the Xantrex and try that. It may be that you have one bad battery or even one bad cell in a battery.Your math is wrong on the current draw, too, if I'm understanding you correctly. If you draw 7 amps at 120V, the draw from your batteries will be in the order of 70 amps at 12VDC. At that level of draw, you would be at 50% of battery capacity within 3 hrs.

thank you, I just saw that capacity was 200 for each pair, but not getting the 7 amps to 70 amps vdc but I am sure you are right. I actually have isolated each "pair" and they both do the same thing.am I correct in assuming that going below 12.3 volts is less than 50%?

What size of battery cables are you using to connect the batteries to the LOAD.70AMPS DC current is a pretty heavy draw fro the batteries.Roy Ken

Your likely not starting with fully charged batteries. Taking the bank to 13.6 is nowhere near fully charged from a discharged state. You need to hold those batteries at 14.8 volts for at least 2 hours and that would likely get to 95%+ full.Also pulling around 70 amps DC (7 amps AC through inverter) is going to cause battery voltage to dip pretty low. That's a significant power draw. I would fully expect battery voltage to drop well below 12.0 volts with several minutes at that power draw. Under ant significant power draw, battery voltage is not a good indicator of charge state. Battery voltage charge state estimates is only good at zero draw on the battery

well, that is interesting. I have a xantrex true charge 40 (microprocessor controlled) and it brings the batts up to 13.4 and keeps them there(green ready light is on). if I shut it off, the volts drop to 13.3if it will only trickle charge at 13.4 volts, how do I get them up to 14.8 volts

6 volt battery issue question about load | Good Sam Community

16 Replies

MEXICOWANDERER
Explorer
May 12, 2014
:h

Sorta confused here. Lots of info from the OP but all of it is in fact irrelevant as far as determining if a battery is good or not good. Charge the batteries, run a hydrometer test, then if there is any doubt verify with a adjustable carbon pile load test.

Flooded lead acid and Absorbed Glass Mat are two INCOMPATIBLE technologies at the very second they stop being charged until they start being discharged. They cannot be left connected, just sitting there. Connect them together when they are being charged, or discharged as one bank. Otherwise separate them by disconnecting with a battery switch.
wa8yxm
Explorer III
May 12, 2014
There are two explanations for sudden voltage drop under load.. Only two.

Either

1: Batteries have developed higher than normal internal resistance (This happens as they get older) =or=

2: Bad connection external to or on the battery (Also very common)

HOW to figure it out.

Get a good digital volt meter, place probes ON THE BATTERY TERMINALS and note the voltage, compare to voltage on Xantrex remote.. Should be identical but if they are not,, Just note difference (As a percentage)

Now apply load.. Again compare voltages.

if you need help with the math post results and I'll figure it out

but if the batteries remain in the 12.6 range while the Xantrex shows 12.0 or less, Connection issues.

If the voltages remain close to the same (NOTE: there is always SOME loss) battery condition.

My chassis battery was replaced either Dec or Jan.. When I took it to the battery store I told him to go ahead and test it (Dang thing would not start the MH any more had to jumper in house batteries (BOOST SWITCH) to get'er done).. I was told the battery tested "Full charge" but, only 50% cranking amps. (IN short, high internal resistance).

One of the standard failure modes.

NOTE: under light load, the battery was perfect.. But under heavy load... Well.
westend
Explorer
May 12, 2014
One thing to add: Conventional 6V GC2 batteries like a big charge going into them, around 15V and at least 10A for a pair. Mexwanderer has covered this a couple of times. This is to get the most out of them and increase the longevity.
2oldman
Explorer II
May 12, 2014
rockwind1 wrote:
a real load on them (like pulling 7 amps thru the xantrax 1800 inverter),
Others assumed (and perhaps correctly) that you mean 7 amps @ 120v is coming OUT of the inverter. That would qualify as a 'real' load and require 70+ from the batts. Yes, that will draw them down.
rockwind1 wrote:
from what I understand, going down to 12.3 volts, you have essentially used 40-50% of the battery capacity?
Not in the situation you describe. That figure represents resting voltage, which would occur a few minutes after your 70 amp draw is completed. 4 Deep-cycle batteries struggle to produce that much amperage, compared to a car battery which probably would not.

Here's a recent thread on what you're doing:
Mix AGM and acid 2013
BoonHauler
Explorer
May 12, 2014
You need to use a Hydrometer and check the specific gravity. This is the only 'real' way to check a FLA battery.

You should be looking for 1.275+ SG reading for 100% SOC.

A good hydrometer is this one
pianotuna
Nomad III
May 12, 2014
Hi,

Voltage drop under load is normal. It is greater with the thicker plates of six volt jars.

Test the load for five minutes. Measure the voltage 30 minutes later.
CA_Traveler
Explorer III
May 12, 2014
rockwind1 wrote:
thank you, I just saw that capacity was 200 for each pair, but not getting the 7 amps to 70 amps vdc but I am sure you are right. I actually have isolated each "pair" and they both do the same thing.

am I correct in assuming that going below 12.3 volts is less than 50%?
7Ax120V=840watts. You have to draw 840W out of the battery. So 840W/12V=70A. You likely lose 10% in the inverter so you're drawing about 77A.

After drawing a load the batteries have to rest with NO loads for about 2 hours in order for the voltage to accurately reflect SOC. When did you measure the 12.3V and any loads at that time?

You have unknown batteries. Take them to a good battery shop for load testing.

A voltmeter will only tell you so much about SOC. A battery monitor provides a much better understanding.

I agree with others that it doesn't seem like they are being fully charged. The Truecharge is an excellent charger, however, so something doesn't seem right.
motorcycle_jack
Explorer II
May 12, 2014
I don't know how this get's in the DIY category but it is. Also I don't know how so much misinformation on batteries is given here. Two 6V batteries in series does NOT double the current capacity, it double the voltage. Two 12V batteries (if the same AH rating) double the current available. Also the maximum drain you should attempt before killing the battery is defined by Peukerts Law (about a 20 hour drain amount). So a 200AH rating should give a max drain value of 10A - DC amps that is. Ten amps of DC will convert to 1A of AC power (P=IE must be equal for supply and drain). 10 X 12= 120 so 1 X 120= 120. If you are drawing 7A AC you are killing your batteries right off! That equals 7 X 120 = 840 so 840/12= 62 AMPS DC!! An inverter providing that much power needs more batteries to supply it (about 6 in this case).
rockwind1
Explorer II
May 11, 2014
well, the xantrex starts at 40 amps and charges at 14.8 volts up to 80 or90 %, then gradually switches down to a float charge.

are you saying that the resting voltage of a good battery without being charged would be 14.8? I thought a fully charged battery would be at about 13.5 or .6?
RoyB
Explorer II
May 11, 2014
I will add more to what hmknightnc says that you can get the three banks of 6VDC batteries to around 90% charge state in around three hours charge time if you hit them with 14.4VDC @ 55AMPS or so of DC charge current. Each group of 6VDC batteries will want to draw around 18AMPS of DC Current when hit with 14.4VDC according to Progressive Dynamics.

Only using 13.6VDC charge voltage it will take over 12-14 hours to obtain a 90% charge state of the three battery banks..

Battery Charging science at work here...

Roy Ken