Whats normal voltage drop from a 4amp load? (210ah)

You are right, that is too much of a drop for that small load. Indicates the batts are not fully charged like you think from using that charger. You must use an hydrometer to determine the actual state of charge.

You might not have a charger that can do it right.

Pretend your 6s are Trojans (be similar for this work) and follow Trojan's instructions here

https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf

do automotive battery tenders actually charge?

i have a 200amphr AGM, This Morning i have warmed up several cups of coffee in the MW and heated my jimmy deans breakfast bowl in the MW, i am using this PC, battery status is 24 AmpHrs used, 88% SOC remaining, 6 amp draw , voltage 12.63v

your voltage
that is too much voltage drop

batteries were NOT fully recharged

tenders are designed to maintain a battery that is fully charged
NOT to recharge a battery that has been used
voltage from a tender is NOT high enough to fully charge "any battery"

In a way you could argue the opposite regarding battery tenders and deep cycle batteries.

Its a good safe way to charge the battery. Reputable company i feel safe plugging it in and walking away.

Not a good way to maintain because isnt it around 0.5v too low for flooded?

wopachop wrote:
It's a good safe way to charge the battery. Reputable company i feel safe plugging it in and walking away.

You're going to have to tell us the amperage of the charger and how long you left it on. The week?

wopachop wrote:
Not a good way to maintain because isnt it around 0.5v too low for flooded?

I don't understand this. You mean .5 amp?

wopachop wrote:
In a way you could argue the opposite regarding battery tenders and deep cycle batteries.

give us your best shot.

you got the issue and you are using the gadget!

wopachop wrote:
I have two brand new 6v from costco. I pulled 20amp hours out of them a week ago and then fully charged with an automotive battery tender.

Today i put a 3.8a load on them and voltage is down at 12.15v after a few minutes.

Are they supposed to sag that much under a little 4amp load? Did a quick search and all im seeing is topics about voltage sag while under solar power. My batteries are on the garage floor.

edit: I should have checked early. My watt meter is saying 12.15v.

But 2 multimeters are saying 12.27v and 12.3.

What "tender" are you using??

Most battery Tenders are pretty darn wimpy on output, .5A-1.5A MAX.. Nothing more than a high priced glorified "trickle" charger.

At that rate it will take MONTHS to fully recharge your batteries..

From Battery Tenders manual for their 1.25A maintainer it says..

"TIME REQUIRED TO CHARGE A BATTERY:
The Battery Tender® charges at a rate of 1.25 Amps or 1¼ Amp-hours per hour. Therefore, a fully discharged 15 Amp-Hour battery will take approximately 9½ hours to recharge to 80% capacity. "

You need to wallop them with a charger with considerably higher amperage capacity than a battery tender..

Battery tenders are not designed to "recharge" a DISCHARGED battery, they are only used to MAINTAIN A FULLY CHARGED BATTERY! In other words battery maintainers are there to prevent your battery from self discharging..

There are off brand "maintainers" which offer higher amperage capacity than the original "Battery Tender" but since you have a RV, why bother, your RV already HAS a high current charger built in (the converter), why not let your RV converter do the heavy work??

wopachop wrote:
I have two brand new 6v from costco. I pulled 20amp hours out of them a week ago and then fully charged with an automotive battery tender.

Today i put a 3.8a load on them and voltage is down at 12.15v after a few minutes.

I am not convinced the batteries are fully charged. Did the charger hold 14.7+ volts until amps charging was less than one amp?
Have you verified full charge with a hydrometer?
Did you let them rest unconnected 48 hours and observe 12.7+ volts?

You can "feel safe" and screw it all up, or you can "be safe" and do what it says in that Trojan link above.

I think the battery tender goes to 14.3 or 14.4v while my flooded batteries want 14.8?

Pretty sure the battery tender is 5amp. Im using it as a temporary charger to cycle the batteries in the garage once a week. Bought the new batteries for a camping trip back in Feb and didnt need them. Should have waited as im plugged in most the time.