Expected voltage sag?

Hi Chandalen,

That depends on the load, and size of the battery bank. For example my 3000 watt inverter draws 300 amps when running flat out. That taxes the 556 amp-hours of AGM batteries--so there is voltage drop far in excess of 0.1 volt. The wire is correctly sized for 300 amps.

Chandalen wrote:

Anything more than 0.10v is too much.

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A dip with the hydrometer might turn up the culprit. At the least, it will show that all the cells are fully charged.
If you've been nursing them along and they have solar every day, it would seem that the batteries should be OK.

How long did they sit uncharged?

Beautiful blues and greens?
OP update:
I opened up the battery cases today and found that the corrosion was back, this time in lovely green to go along with blue. I cleaned all that gunk off and scraped things with a pocket knife and sandpaper until I had nice gleaming metal at all the connections. I also cleaned up my ground connection to the frame. This seems to have addressed the voltage drop, with the panels off and inverter on with the same laptop charger load of just under 100 watts I'm only getting a small voltage drop. It seemed to level out at around 12.4V. Once again, voltage measurements are taken right at the posts via the SCC voltage sense wires. I dipped the hydrometer in each cell and the readings were all right around 1.300 with panels disconnected. Does that seem OK? Seems too high, although the SCC was in float when I started the maintenance. Anyway, we'll see how things go now with cleaned up connections.

First battery in series, lovely greens and blues:


Close up of the green gunk:


Second battery in series:


Cleaned up:

Put honorable meter negative probe on negative post. Put positive probe tip anywhere on the PCV lid of the battery. Do you get a voltage reading? If yes the lid is polluted with electrolyte. Wet between the cells is a nice short both n distance and effect. Adds up during months of disconnected storage.

This has yet to be discussed but sprinkling a little baking soda on the lid between cells is an excellent preventative measure. Four months 5 ma = 14.4 amp hours needless discharge and a disconnect switch does not do a thing to prevent it.

I tried testing the PVC lid with the multimeter as you suggested and got a flickering from the meter. It never exceeded 0.1V.

I repeated my laptop overnight test, although I've picked up a 12V power cord for it since the last time and that is what I used. Before bed the voltage was 12.4V. This morning it was 12.2V and returned to 12.4V when I unplugged the laptop charger. Outside temp is just above freezing at 35F.

I'd definitely get out the baking soda and some hot water. Give the battery tops and terminals a good bath. Let dry and then cover the terminals and exposed cable ends with dielectric grease or a terminal spray.

With that much oxidation going on, I might put a penny under the cables and/or punch some holes in the case lids for air exchange.

how about using some pookey to seal the plastic lid/battery posts? or is this just leaking from the fill/check holes?

I don't know where it is leaking from and it had not occurred to me before that it was. I wonder if the semi-frequent equalization cycles that I put the batteries through when trying to recover them caused the corrosion from venting? Maybe normal usage would not? I put them through 2 or 3 temp-corrected 15.5V equalization cycles this winter with a max voltage setpoint at 16.15V. It usually ran the voltage right up to that max setpoint with the temp correction (it was winter in Montana after all).

Clean the top of the battery. Make the connections. Cover the connections with a battery protectent spray available from General Motors parts supply stores.

So I pulled things apart again, baking soda cleaning and scraping of corrosion again, then reassemble with dielectric grease and protectant spray. I also cleaned up the corrosion on my cable ends and applied liquid tape. Hopefully I'm good!

Cleaned up pics.