expected normal voltage drop from load?

[ewarnerusa]

Is there a rule of thumb or guidance for an expected voltage drop based on amp load on a pair of 6V GC batteries? I was tinkering in my trailer this weekend and ran a 750 watt AC electric heater off of my inverter. Batteries seemed fully charged prior to this based on voltage readings, but no SG reading taken. I had a much larger voltage drop than expected, it was pulled down to 10.5V and the inverter was giving an alarm. I assume due to low voltage. Although the whole time the heater was operating and I was getting hot air out of it. I let the heater run for only about 15-30 seconds. Ambient temp was around freezing (32F). I only did this experiment out of curiosity and not out of any need. Sometimes I'm just bored and want to tinker in the camper! Inverter wired with 0 gauge wiring, about 5' from batteries. Voltage came back up to about 12.5V after removing all loads.

I think I've done this same thing before and did not have such a large voltage drop. Due to the trailer being in the repair shop all summer and the batteries being in various states of disconnect/connected but no solar/solar on but parked inside, I think my batteries took a bit of a hammering. They were like 10.5V when I hooked up the trailer to bring home. I know that's totally dead for a 12V system, but they are deep cycle and this is the first time they were ever down that low. Batteries are 2 years old. I was going to wait until spring to replace them if needed, but I also thought they seemed to be behaving fine after several days of shore-power charging and then letting my solar take over. But it does seem like they are showing a larger voltage drop for small draws (like LED lights) than I remember before this summer.

[CJW8]

I had a similar experience when I had two costco GC batteries in my previous camper. Voltage sag to below 11V with 750-1000 AC watt load on inverter.

Currently I have 3 Group 24 so I decided to run the same experiment. I shut off the charger, turned on the inverter, powered up a 750 watt electric heater. Batteries were at Float voltage so when I turned off the charger so I gave them a few minutes to settle down to 12.8 volts before I turned on the heater. I ran the heater for 1 minute. Voltage sagged and stabilized in the last 15 seconds at 11.84. Then I turned off the heater and voltage rebounded to 12.5 volts.

I am not a fan of group 24's but that is what fits in my battery space. However, they did perform better that the 2 GC's under high load conditions. The 3-24's are roughly equal in capacity to the 2-6's

[mchero]

Ed, Next time you put a big load on those batteries like you stated above immediately go out to the batteries and feel all cables & connections, Be careful, something "might" be hot.
Quick & dirty way to check for bad connections or undersized cables. When I purchased the new to us Discovery I found a battery interconnect was hotter than a Chernobyl reactor core.
I purchased four Sam's Club golf cart batteries back in 05. The person I sold the prior rig to still using the same batteries! What I think the key here was I also replaced the converter to a Progressive Dynamics 60 amp smart charger with the charge wizard. I kept them watered, NEVER let them fall below 12VDC. First 8 years she was NOT plugged in during the New Hampshire winters. I'd fully charge the batteries and then hit the disconnects.
Treat them right and you will get years of service out of them.

[DrewE]

BFL13 wrote:
"Now I understand that when a battery is up in absorption voltage range, the amps required to hold Absv keep tapering. If the amps do not taper then voltage will just keep rising"

This concept still bothers me. I don't believe the amps keep the voltage up.

The charger voltage is kept up by itself. The battery voltage is based on the SG of the electrolyte. The difference between the two voltages makes for amps across the R.

If the charger voltage is maintained amps will taper as battery voltage rises.

I don't think that if you somehow dropped the amps (by having a lower amp current-limit charger?) but kept charger voltage steady, that battery voltage would suddenly drop too. I think it would just rise more slowly from then on.

I think LY has the tail wagging the dog, but I could be missing something in all this.

The charger just plain can't "drop the amperage" without simultaneously lowering the output voltage at a given state of charge, etc; that's physically not possible. A given applied voltage implies a certain specific current flow, and vice-versa.

If the charger is operating in a current limited mode (or is a constant-current supply, which is more or less the same thing), then the voltage is being regulated to produce that current. If it's operating in a constant voltage mode, then the current is basically being regulated to meet that voltage. (Technically, in either case, it may be slightly more correct to say that the charger regulates its power output to achieve the desired current or voltage setpoint, but that's splitting hairs.)

[BFL13]

"Now I understand that when a battery is up in absorption voltage range, the amps required to hold Absv keep tapering. If the amps do not taper then voltage will just keep rising"

This concept still bothers me. I don't believe the amps keep the voltage up.

The charger voltage is kept up by itself. The battery voltage is based on the SG of the electrolyte. The difference between the two voltages makes for amps across the R.

If the charger voltage is maintained amps will taper as battery voltage rises.

I don't think that if you somehow dropped the amps (by having a lower amp current-limit charger?) but kept charger voltage steady, that battery voltage would suddenly drop too. I think it would just rise more slowly from then on.

I think LY has the tail wagging the dog, but I could be missing something in all this.

[ktmrfs]

BFL13 wrote:
The other question is whether it is worth the bother to do a recovery when you can buy 6s in the States for $90 each instead of $180.

Before they can be treated "normally" again they need to be recovered properly which is a big effort and maybe not possible for a busy person.

I get about 0.8 drop on my four 6s with 110a of load. So to stay above 11v alarm I need about 12v going in for the drop and a bit left over for actually running the load for a few minutes. A PSW inverter would want more like 150amps to run the same MW, so that would make for a bigger sag, so you would need 12.x going in.

With less of a battery bank, the voltage needed going in is higher, so with two 6s you need to be near 75% SOC instead of 50% SOC as with four 6s.

your results/recomendations for GC is very similar to what my experience has been. Running a microwave with 75A draw on a pair of GC, is marginal. while with 4 easy to run it down towards 50%SOC.

That's IMHO the major downside to GC. you trade of something for the long life and ability to take hundreds of cycles down to 30% SOC or so.

[landyacht318]

ewarnerusa wrote:


Thanks for the voltage and current relationship description.

YOu are welcome.

I remember years ago before understanding this myself, thinking that volts and amps were two adjustable entities separate from each other, but somehow mystifyingly related.

Now I understand that when a battery is up in absorption voltage range, the amps required to hold Absv keep tapering. If the amps do not taper then voltage will just keep rising.

One does not want to blow past absorption voltages with an EQ charge until the battery has first had a few hours at ABSV and amps tapered to low numbers.

Mex says to apply 5 amps until 16v is reached doing an EQ. I was just setting voltage at 16v and whatever it took amperage wise to get there, so be it, but if it was over 6.25amps(5% of capacity on a 130Ah battery) I backed off and gave it more time at absorption voltage before bringing it again to 16v and waiting for SG to max out.

I got ~ 500 cycles to an average 50% to 60% SOC from that battery, which is respectable for a 12v marine battery.

[ewarnerusa]

0 gauge series connection wiring on my batteries.

Thanks for the voltage and current relationship description. It confirms what I knew about how my solar charge controller is doing it. But for some reason I was thinking some other magic could happen when using a stand alone charger plugged into shore power.

[2oldman]

full_mosey wrote:
What is the typical Amp draw per battery in Golf Cart operation?

Looks like the motors are capable of 8hp, which is a whopping draw even at 48v. Probably more like 3hp which is around 50a at 48v.

[landyacht318]

What gauge is the series interconnect on those GC's?

In my experience, the more abused a battery is the longer the EQ cycle needed.

I was rather timid in my EQ charging, but ultimately found that 16v could do in 45 minutes what took 15.5v 4 hours to accomplish.

Any charging source produces only enough amps to reach/maintain the desired voltage once it gets battery terminal voltage in the absorption voltage range or higher.

If it only takes 2 amps to hold 15.5v, then 2.5 amps would push voltage up beyond 15.5v. One can perhaps limit amps to say 5 and see how high that amperage can push battery voltage, but one cannot say I want 10 amps at 16 volts if the battery only needs 6.2 amps to be brought upto and held at 16v.

My single flooded group 31 would require about 1.4 amps to be held at 14.9v after 3 to 4 hoursat 14.9v, and then ~ 6.2 amps would be required to push the battery to 16v. When amps had tapered to about 4.2 at 16v, SG was usually maxed out. This generally took 45 minutes to 2 hours, depending on the ambient temp, and the amount of abuse the battery endured since the last EQ charge.

Right before I removed this battery from RV service, more than 6.2 amps was required to attain 16v, and amps would taper only to 5.5, then start increasing again as battery temperature started increasing rapidly.

This behavior, along with the bottom of one cell closest to the (-) getting much hotter than the rest of the battery when charging at a ~8 to 9% rate, were the indicators of the end of its cycle life.

This battery has survived 6 more months so far of fairly shallow cycles and no maintenance and no EQ charging in a stationary duty. I estimate its total capacity to be 45 to 50% of what it had when new.

Voltage sag under load is pretty extreme, as is its self discharge. My hydrometer is covered in dust, As the battery I do care about and cycle is an AGM, that continues to impress me, as long as it is high amp recharged and fully recharged each cycle. Low and slow solar only recharging and achieving only 95%SOC and it is not impressive in terms of voltage held under load for AH removed.

[full_mosey]

Canadian Rainbirds wrote:

2oldman wrote:
From my experience, 2 GCs are not enough battery for large draws. 4 is better, 6 is real nice.

Even our 4 GCs show a large drop when running our micro/convection oven. Down to low 11s. And they are connected with just few feet of 4/0. We don't run the mw too long!

Typical for GCs. They are optimized for, well, Golf Carts. What is the typical Amp draw per battery in Golf Cart operation? A MW puts you in CCA land, a spec you rarely find in a GC battery.

I ran an 800W load for 1:15 from a pair of 135AH AGM 5yo telcom pulls. The lowest Volts reading was 11.99 with 7ft of 2/0 cables.

HTH;
John

[BFL13]

The other question is whether it is worth the bother to do a recovery when you can buy 6s in the States for $90 each instead of $180.

Before they can be treated "normally" again they need to be recovered properly which is a big effort and maybe not possible for a busy person.

I get about 0.8 drop on my four 6s with 110a of load. So to stay above 11v alarm I need about 12v going in for the drop and a bit left over for actually running the load for a few minutes. A PSW inverter would want more like 150amps to run the same MW, so that would make for a bigger sag, so you would need 12.x going in.

With less of a battery bank, the voltage needed going in is higher, so with two 6s you need to be near 75% SOC instead of 50% SOC as with four 6s.

[Canadian_Rainbi]

2oldman wrote:
From my experience, 2 GCs are not enough battery for large draws. 4 is better, 6 is real nice.

Even our 4 GCs show a large drop when running our micro/convection oven. Down to low 11s. And they are connected with just few feet of 4/0. We don't run the mw too long!

[2oldman]

From my experience, 2 GCs are not enough battery for large draws. 4 is better, 6 is real nice.

[ewarnerusa]

Charge controller has temperature correction, although I do have a HVD setting at 15.95V. I'll up my equalization time/voltage parameters and HVD, but I'm not sure that will actually get me more time at that voltage during short winter daylight. I also wonder if the solar equalization is a "proper" one with this PWM controller (Morningstar Tristar 45)? It holds the voltage by controlling amps, so while the batteries are being held at that voltage it is just getting a trickle of amps. Is that really doing what it needs to?

While I have a hydrometer, I do confess to laziness in checking it during winter and particularly since 12" of fresh snow that fell since Sunday. The last time I had the batteries opened was late fall when I topped off water. I took an SG reading then, but it was during active solar charging so I don't think it was a legit reading. Does SG get biased during active charging? Is it impacted by a surface charge?

I certainly don't doubt that my batteries have been compromised and will replace them if necessary. My solar charge controller is completely customizable, so I'm open to charging parameter suggestions as well for use during winter storage. The only battery charger I currently have is the onboard WFCO 55 and the sun.