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AGMs and Inverter DC Voltage Sag -More Results

BFL13
Explorer II
Explorer II
More on how AGMs do well under high amp draws for minimal voltage sag.

Recently put two 100AH AGMs in the truck camper and changed the little 120v 3.2 cu ft fridge out for a new to us Dometic 7401L. Have a 1000w Sharp microwave in the TC that came out of the 5er in another shuffle.

We have had threads before showing how AGMs do better than Wets for this job and how you can have two AGMs (which is easier in a small RV) where it would take four 6s for that smaller voltage sag- if you had room. So I did a test to see just where things are now in my TC.

I plugged the TC shore power into the 5er's MSW inverter so I could read the TC amps from the 5er's Trimetric. MW drew 117a DC and the fridge on 120 drew 13.4 DC amps.

Now plugged TC into TC inverter on the two AGMs (200AH). 1.0 voltage sag with the MW on presumed 117a. Got 0.1? sag not sure anything happened with the fridge at presumed 13.4a.

For comparison in the 5er with inverter drawing from four wets (430 approx.) and similar MW, I also get a 1.0 volt sag.

There are some differences in wiring etc, but for a general comparison, IMO this proves what has been stated before by those who have a couple of AGMs in their smaller RVs and are able to run high amp items no sweat.

The AGMs cost more, but for this kind of work they are perfect. Which is lucky, because they have to be inside our TC so we can't use Wets with their nasty battery gasses anyway.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.
12 REPLIES 12

BFL13
Explorer II
Explorer II
I should have mentioned the type of AGM. Yes 5 whatsits. (But with two isn't that in half?) ISTR a regular batt is about 10 whatsits from when Salvo was measuring Rs.

http://www.hespv.ca/hesproductspecs/HES/Stark_Battery_TD_ENG.pdf

Battery made in China. Seems to be the same one as that 12100. Working well in my set-up, no complaints. Recharge and Float voltages are right for my converter too (14.8 Vector charger/13.7 float on the7455.) I can do both in parallel with a 55 amper and be close to their 27amp each spec.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.

landyacht318
Explorer
Explorer
It should also be notes that BFL's AGM's do not have as low a resistance of many other AGM's at ~5 m OHM.

https://www.wholesalesolar.com/cms/upg-ub-121000-agm-battery-specs-753736796.pdf

A similar Sizer Odyssey/Northstar AGM has ~ 2.6m Ohm, a Lifeline AGM ~3.2M ohm.

What i love about my Northstar is if it wants 100 alternator amps, it will gobble them up without worry.

As far as state of charge, well an ammeter is a wonderful enlightening tool.

MEXICOWANDERER
Explorer
Explorer
Solution.

(3) 1500 watt 16 volt TVS devices
(1) .01 uf capacitor
(1) .82 uf capacitor
(1) 1,000uf (min) 25 volt electrolytic capacitor
(1) Hyperfast freewheel diode

Connected to vehicle IGNITION main power circuit.
Inductors in vehicle circuit are isolated from becoming a tank-circuit because all of them are isolated by vehicle's relays.

Some of you have my filters. Aren't you glad?

MEXICOWANDERER
Explorer
Explorer
CARE CARE CARE CARE

Must be taken if augmenting inverter MICROWAVE OVEN draw by running a vehicle engine.

The magnetron in the microwave manages to couple with the inverter's FET power transistors when the inverter magnetron saturates (startup) or collapses the field (shutdown).

Techhies can connect their oscilloscope to the 12 volt primary (do it at the vehicle chassis IGNITION connection). Dramamine recommended.
The transient voltage spike would make popeye the sailor seasick.
The problem is the coupling of the vehicle's battery with vehicle ignition power because the circuit is made complete with the ignition key in the "on" position.

BFL13
Explorer II
Explorer II
Using Wet Starting batteries for the fairly brief high amp draws instead of Deep Cycle AGMs is sort of clever IF you do have a way to recharge them right away.

Although vehicle batts sit there being drawn down by parasitic loads for days and days while also being under re-charged on short trips around town, they last quite a while. (Even without being put on a trickle charger between short trips.)

Hard to arrange an early recharge in ordinary RV situations where you need Deep Cycle batts that can take some 50-90s in a row before seeing 100 again. Maybe if you had lots of sunshine every day for sure.

Very little price diff between Starting and Deep Cycle Wet batts though.

If you had dedicated Starting batts for the big inverter draws you would still need Deep Cycle batts for ordinary camping. You have space for X batts, usually four.

Four 6s will do the job so no need for the Starting batts, but if you have only space for two batts, the AGM advantage for voltage sag comes into play, but if you choose Starting batts for that to save money vs AGMs, where is your Deep cycle 50-90s ability going to go? No space.

Two of those RV/boat 27s that are mostly starting batt might be a good choice --except they are such a PITA to recharge to 100% after doing a few 50-90s. BTDT--Never again!!!!

So, yes. If you really knew your scenario, you could pick just the right sort of battery bank(s) to suit
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.

SoundGuy
Explorer
Explorer
BFL13 wrote:
The point here is just the AGM's ability to stay above the inverter's 11v alarm for those AH before getting down to 50%.


A worthwhile test for sure - thanks for posting your results! 🙂 That said, for those of us who only intermittently use our inverter there is an alternative I've chosen - load supporting my single G27 wet with the truck (running). Without it my 750 watt toaster would drag down voltage to where the inverter would almost for sure alarm even with a full charge on the battery, with load support from the truck voltage sits around 13.2 volts, allowing the inverter to handle the load without issue. Certainly not a solution for those who use an inverter regularly but for intermittent use it is far less costly than investing in a pair of AGMs.
2012 Silverado 1500 Crew Cab
2014 Coachmen Freedom Express 192RBS
2003 Fleetwood Yuma * 2008 K-Z Spree 240BH-LX
2007 TrailCruiser C21RBH * 2000 Fleetwood Santa Fe
1998 Jayco 10UD * 1969 Coleman CT380

MEXICOWANDERER
Explorer
Explorer
Compromises compromises. There really is a use for that gray matter between the ears 🙂

Thanks BL 13.
I have installed Delco high CCA group 31 (900 cca) calcium calcium FLOODED batteries as a dedicated power source for a high wattage microwave inverter combination. In a big pusher diesel. Works good for a 20 minute defrost cycle too. But care must be used to not let flooded batteries sit longer than overnight before recharging. And review the word above "Dedicated". And MAX amp hour discharge is 15% no exceptions.

BFL13
Explorer II
Explorer II
Reminder that two batts will not last as long between recharges as four so one is not as good as the other for that.

The point here is just the AGM's ability to stay above the inverter's 11v alarm for those AH before getting down to 50%.

Another way to look at it is with 200AH of Wets you have 100AH but the inverter will alarm on high draws once down to 75% SOC, so that gives you 50AH of window, while the 200AH of AGM lets you use all 100AH of window for occasional high amp draws. You still have 50AH left of Wets between 75-50% SOC but can only use that for lower amp draws.

You need four Wets to do that, but now you also have a 200AH window for occasional high draw use instead of 100AH with the two AGMs

So if you are going to carry four batts anyway, might as well save money and use Wets, unless you need to keep the batts inside. The AGM advantage is where you only have room for two batts.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.

ktmrfs
Explorer
Explorer
yup, you hit on possibly the biggest drawback to wet cell GC batteries. they are NOT a good choice for high current draw. And you hit on possibly the biggest advantage of an AGM battery.

Boils down to carefully analyzing YOUR 12V use conditions and picking the best battery choice for YOUR applications.

Seems like you've done a great job and a great solution.

And thanks for the direct comparison. Good validation of the differences.

In our case I have room for 4 GC, perfect solution for 90 percent of our use on long dry camping times, and adequate for the occasional running of the panasonic true inverter microwave at 1000VA for occasional 5 minute or less times when heating veggies, warming coffee, etc. But it really takes 4GC to run even 1000VA off the inverter for SOC below about 85-90 percent. Two just won't cut it, way to much voltage drop.
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2015.5 Denali 4x4 CC/SB Duramax/Allison
2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!

BFL13
Explorer II
Explorer II
pianotuna wrote:
BFL13,

Is this 1 volt DC voltage sag?


Yes. Using my 2000w Vector MSW and whatever wiring. I don't mind that amount. 12.2 would see 11.2 which is still above the 11v alarm if the batts are ever that low. In real life, with solar the batts are close to full.

Now with the propane fridge our battery worries are over. That little res fridge on inverter was killing us on cloudy days.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.

pianotuna
Nomad II
Nomad II
BFL13,

Is this 1 volt DC voltage sag?
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

OldSmokey
Explorer
Explorer
BFL13 wrote:
More on how AGMs do well under high amp draws for minimal voltage sag.

Recently put two 100AH AGMs in the truck camper and changed the little 120v 3.2 cu ft fridge out for a new to us Dometic 7401L. Have a 1000w Sharp microwave in the TC that came out of the 5er in another shuffle.

We have had threads before showing how AGMs do better than Wets for this job and how you can have two AGMs (which is easier in a small RV) where it would take four 6s for that smaller voltage sag- if you had room. So I did a test to see just where things are now in my TC.

I plugged the TC shore power into the 5er's MSW inverter so I could read the TC amps from the 5er's Trimetric. MW drew 117a DC and the fridge on 120 drew 13.4 DC amps.

Now plugged TC into TC inverter on the two AGMs (200AH). 1.0 voltage sag with the MW on presumed 117a. Got 0.1? sag not sure anything happened with the fridge at presumed 13.4a.

For comparison in the 5er with inverter drawing from four wets (430 approx.) and similar MW, I also get a 1.0 volt sag.

There are some differences in wiring etc, but for a general comparison, IMO this proves what has been stated before by those who have a couple of AGMs in their smaller RVs and are able to run high amp items no sweat.

The AGMs cost more, but for this kind of work they are perfect. Which is lucky, because they have to be inside our TC so we can't use Wets with their nasty battery gasses anyway.


AGM has a lower internal resistance so more discharge current.
you trade off lifespan for this.. also, agm do gas if overcharged..

good reason for agm use is spillage.. there is none..
bad is you can't measure SOC accurately.