Battery voltage drop under load?

[Dupdyke]

Electrical engineer I know laughed and told me If you run toaster, Microwave, Elect Bunn coffeepot, hair dryer. You have a generator use it and dont put that much unnecessary load on the batteries. While you are running the gen it will also recharge the batteries. Those devices put a real strain on inverter so why waste its life cycle. Gen is made for that load and will work just fine.

Only my way of doing it yours might work but been at it 38 years and my batteries last way over five years.

[NormD]

Thanks for all the good information!

The Xantrex output which reads 109 vAC on my plug-in-the-outlet magnetic analog meter reads 110.8 vAC on my True RMS meter. Two volts is not a lot of difference. With a longer run on the microwave, and continuing reduction of battery voltage, the analog reading drops two more volts to 107 vAC.

The terminals and wire terminations are shiny-clean. Voltage drop from the farther battery to the wire going to the inverter is 15.8 mV on the positive terminal and 16.6 mV on the negative terminal under full load. This totals 32 millivolts, a tiny part of my one volt drop. Most of the drop is within the batteries themselves. This would imply that even if the voltage dropped to, say, 11.0 volts, the battery would not be harmed because the drop is due to internal resistance, both mechanical and electrochemical. Good batteries should provide quick recovery to near the initial voltage and minimal change in specific gravity. And of course, the load duration must be short enough so that internal heating does not damage the battery. I plan to test the specific gravity before and after the microwave load.

I have written as suggested to Les Schwab. Perhaps they can provide data or a Johnson Controls part number.

[camaraderie]

Originally Posted by mgscott4

Wrong and it is OK to take a deep cycle battery down to 12.0 open circuit volts.
Your looking at a chart for starting batteries not deep cycle. Deep cycle batteries usually have a higher specific gravity close to 1.33 at full charge rather than 1.265 for starting batteries. This raises the SOC on the chart so that 50% SOC is around 12.0 volts as correctly stated earlier. I think 12.06 volts to be exact. However, the only way to really tell the exact SOC is to use a hydrometer to measure the specific gravity.

Sorry...but my table specs apply to deep cycle as well.
Here it is for deep cycle...the same: Battery voltage and state of charge - Energy Matters
and here: Pacific Power Batteries - About Automotive Batteries
and here: View Article

etc. etc.

Of course a specific gravity is the best way to measure state of charge but we are talking here about DAILY use and cycling to 50% charge to maximize battery life. No one is taking multiple daily specific gravvity readings that isn't also gift wrapping their garbage.....so taking it down to 25% on the voltmeter is not what anyone wants to do and THAT is exactly what you are doing if you take it down to 12.0 resting voltage.

I'll bow out of the argument at this point... just wanted to provide the actual links to DEEP CYCLE SOC charge tables from reliable sources for those wondering about the values and sources. Treat your own batteries as you see fit.
I would also in fairness note that these are measures for STANDARD WET CELL DEEP CYCLE batteries from mass market sources found in most coaches as standard or replacement batts. Should you choose to invest in Rolls or Surrettes etc., the values may well be different. Also if you have AGM's ...you probably need to add .1 to the values (i.e. 12.3 =50%)

[mgscott4]

I think that some of the misunderstanding here is about real time voltage while discharging the connected batteries, versus open circuit voltage, and after batteries at rest (2-6 hours).

There some charts here that show the effect of discharging and SOC.

https://www.google.com/search?q=lead...ih=705&dpr=0.9

In real time use, while boondocking, I can slowly discharge my batteries down to 12.0 volts indicated on my battery monitor, then charge them up with little to no effect on the health of the batteries.

That is because, if I were to discharge them to 12.0 on my battery monitor, then disconnect them, and let them rest for 2-6 hours, then measure the open circuit voltage they will read around 12.2-12.4 volts, which is around 50% SOC.

So, 12.0 volts connected, in real time use is around 50% SOC.

It should also be stated that temperature and rate of discharge are going to effect the voltage level and SOC.

I don"t know to many people that are going to disconnect and rest their batteries to measure the open circuit voltage before charging them up. I don't know too many people that are also using a hydrometer on a regular basis to make sure they don't discharge their batteries too much either.

I agree that batteries that only use the top 10-20% of the battery will last a lot longer than a battery that is used to 50% regularly. Batteries should also be recharged and not left to sit at any discharged level.

I too am bowing out of this discussion, I would also in fairness note that these are measures for STANDARD WET CELL DEEP CYCLE batteries from mass market sources found in most coaches as standard or replacement batteries.

The next controversial topic should be-- Is it safe or necessary to charge wet cells to 14.8 volts or just limit to 14.4 volts to fully charge your batteries? (Temperature Compensated). lol There is a lot of discussion about this in the Solar community.

[jauguston]

To start the 14.4 vesus 14.8v discussion my Fullriver 6v AGM house batteries recommend a maximum of 14.9v charge voltage for a 12v pair (-:

My Magnum Energy Inverter/Converter and my Blue Sky Solar controller both have a 14.8 setting that I use.

Jim

[clyon51]

When we got our present rig, it had Trojans. Trojan said something like 14.7v recommended for boost mode. I later upgraded the charger to a Progressive Dymamics PD9245, which does 14.4v on boost and desulf mode.

Back then (and a lot dumber) I asked if I could get one with a 14.7 boost and of course they said no. They explained that although the Trojans can handle the 14.7, they far from NEED it to fully charge. It's just with those extra 0.3v, they will reach a 90% SOC a little bit sooner.

They further explained an automotive charging systems uses a max of 14.5v, which any battery should handle, so they keep within that spec. Made sence then and still does.

I haven't read the thread(s) where the super boondocking solar folks are discussing these extra tens of volts, but I can certainly imagine

It certainly makes sence though they want to squeeze 100% out every aspect of their system and certainly reaching 90% SOC faster is better and cheaper when using a genny.

I know nothing would please me more than to have a system where I could pull into our regular winter GC (which charges $0.16 @ kwH) and never plug in! Just not cost effective and the fact we don't boon that often, at least not yet. We are very happy going 3 days without a recharge now with no solar.

BTW mgscott, I'm with you 100%.

[jauguston]

A lot of newer cars have a maximum charge voltage of 15v+.

Jim

[clyon51]

Originally Posted by jauguston

A lot of newer cars have a maximum charge voltage of 15v+.

Jim

And what kind of batteries do those have? I know the DWs 2012 Equinox doesn't have an AGM and the output from the alt still maxes at 14.5v.

[bartlettj]

For heavy load inverter applications, a hybrid start deep cycle is probably the right choice if it has enough capacity. Golf cart batteries are designed for 75 amp burst discharge max per cell, so if you want to use 2GC batteries you will need to run 4. Starting batteries are designed to handle the high current but shouldnt be as deeply discarged as the golf cart cells.