Battery Basics

[rsalhus]

I just read the article on Battery Basics in the Sept issue of Motorhome magazine.  It's pretty good reading but it doesn't cover some things RVers need to know.  For instance, the article contains a State of Charge (SOC) table for lead-acid batteries showing the 12V battery readings when the battery is at 100%, 90%, 80%, etc., SOC.  For RVers, this table will allow you to determine your batteries' SOC from a digital volmeter reading.  For deep-cycle batteries, you know you don't want to discharge your batteries below 50% Depth of Discharge (DOD) before recharging them.  Let's say you're out boondocking, and you've just stayed the first of several nights.  You wake up the next morning and go out and measure your coach batteries and find that they're at 12.25 volts, indicating a 50% SOC.  So you hook up the battery charger to the coach batteries and fire up the generator to charge them up.  But you don't know how long it's going to take to charge them to a full charge, right?  This is what the article is missing.

For me, I've installed a battery gauge in our RV (see the link at the bottom for a picture) which allows me to monitor the SOC of both the coach and chassis batteries.  The Coach Batt/Chassis Batt switch on the battery gauge is a spring-loaded toggle switch that only displays the SOC (in percent) when the switch is held up (Coach batteries) or down (Chassis batteries).  So it won't light up and display anything (and won't draw any current) when the toggle switch is released.  The battery gauge is made by Recreational Technologies, Inc. right here in Minneapolis.

OK, now comes the interesting part.  With the instructions for installing the battery gauge, is a formula for determining how long to charge the batteries based on the batteries' current SOC.  The formula is as follows:

Charging time (hours) = Battery size (AH) X (100 - SOC reading in %) X .01
                                  ----------------------------------------------------------------------------
                                            .7 X Charger current (Amps)       

So an 80 AmpHour battery with a SOC reading of 50% and a 10 Amp battery charger would be:

Charging time (hours) = 80 X (100 - 50) X .01
                                  ----------------------------- = 5.7 hours
                                          .7 X 10   

So I would need to charge my coach battery (I actually have two) for 5.7 hours to get it up to full charge.  Of course, you don't need the battery gauge to determine your batteries' SOC, you just have to measure the voltage and convert it to a SOC value (from the table in the article) and plug it into the formula.  But I can just push a button in my MH and then do the computation needed.  One thing to consider, the batteries should be at rest for at least three hours before reading them.  If you need a copy of the SOC table or have any questions, I can be reached by e-mail.

 

[Ned]

When boondocking, you are correct in that deep cycle batteries need to be recharged when they are at 50% of capacity, but you don't need to charge them to 100%.  We usually just charge to 80% as the last 20% will take the most time.  We use the Link 1000 panel to monitor our battery usage and to decide when to charge and when to stop.

 

[joelmyer]

Rsalhus,

Want to make your power converter act like a $1,000 three stage charger for $25?

http://www.progressivedyn.com/prod_details/charge_wizard_1.html

This puppy gets me up to 80% in an hour or so.

Course, your power converter has to be a Progressive Dynamics!

Joel

 

[Ned]

Not exactly like a 3 stage charger.  I true 3 stage charger begins in bulk mode which is a constant current charge, then changes to a constant voltage mode for the second stage.  The Charge Wizard just boosts the voltage to 14.4v for the 1st stage, so the charging current will taper off as the batteries charge.  Still better than a simple 1 stage charger.

 

[rsalhus]

Ned,

How do you know when you're at 80%?  When the bulk charge stage is complete?

 

[Ned]

I use the AH value from the Link 1000.  I have programmed it for 400AH of battery, when it reaches -200AH I'm 50% discharged.  I then charge until it reads -80AH or less and then I'm at 80% or more of capacity.  In practice, I rarely get to -200AH but I do charge to at least -80AH before shutting down the generator.  The Link 1000 also has a bank of 4 LEDs that can be used as a crude estimate of charge state.  When all 4 turn green, you are at 80+% charge.  That's as good a way to check as any.

 

[John From Detroit]

Rsalhus,

Want to make your power converter act like a $1,000 three stage charger for $25?

http://www.progressivedyn.com/prod_details/charge_wizard_1.html

This puppy gets me up to 80% in an hour or so.

Course, your power converter has to be a Progressive Dynamics!

Joel

That is one of the things I really like about the 2005 Damon I drive....... It is a Progressive Dynamics and they even included the charge wizard.

I've done some research,  I've found a few charger/converters that are, at their best, as good as the PD intella charger with charge wizard.  I've not found one better.  For flooded wet cells that is.  Now for other types of batteries (IE: AGM, not so good

 

[rsalhus]

The Link 1000 sounds like a pretty nice system.  You must have 4 coach batteries with 105 AH each.  I didn't know you could measure the SOC with any accuracy while the batteries were charging.  Do you charge them with a separate battery charger when you're boondocking, and at what amperage?  How long would it take to charge 4 of your batteries from 50% SOC to 80% SOC?

 

[Jeff]

We had an Intelli Charger 9100 on our last Pace and it was the best we had owned after adding a Charge Wizard. Howerver by itself the Charge Wizard will not necessarily charge your batteries in a given amount of time. Intellichargers come in several diferent models starting at 30 amps going up to 80 and the capacity of the charger is the single largest factor in charge rates.

The Charge Wizard helps prevent boiling and venting after batteries are charged and prolongs their life.

 

[Ned]

We have 4 6V AGM batteries of 220AH each, for a total of 12V@440AH.  The Link1000 works with the Heart inverter/charger (we have the 20D, a 2000W model) to tell us the state of the batteries and lets us control the charger and inverter functions from it.  When we're boondocking, we charge the batteries using the generator that powers the Heart charger.  For our configuration, the bulk rate is ~100A so it only take about 1.5-2 hours to take the batteries from 50% to 80% charge.  With our typical electrical usage we run the generator about 4 hours a day.

 

[joelmyer]

Ned,

How do you know when you're at 80%?  When the bulk charge stage is complete?

If you go the Charge Wizard route, you can tell by the light on it.  It stays on solid, blinks fast, blinks slow, blinks very slow.  Each of those translatates to one of the modes: actually it translates to the battery voltage - so it also monitors the state of (dis)charge.

Mount it where you can see it, but not where the blinking will drive you nuts at night.

 

[Karl]

Rolf,

The equation you posted raises a question. What is the .7 factor used for? Also, for that equation to work, the charger must be capable of a constant current output, which means raising the output voltage as the battery becomes charged. I personally don't know of any readily available, off-the-shelf charger that is capable of doing that unless it's a 3-stage 'smart charger'. Another point to think about is that a lead-acid battery should be charged at a float voltage for several hours after reaching 100% to reverse the effects of sulfation and keep the battery healthy. That's why they should be equalized on a more or less regular basis.

I didn't know you could measure the SOC with any accuracy while the batteries were charging.

Correct; it's only an approximation. They should ideally rest 4-8 hours to get a really accurate SOc of flooded cell batteries, but it's not that important for AGM's.

 

[rsalhus]

I wish I could update my converter to the Charge Wizard, but I don't have the one made by Progressive Dynamics.? I think mine is made by MagnaTek and doesn't have a smart charger.? It only charges when hooked up to shore power and probably at a float rate.

The question about the .7 factor in the equation I posted, I believe, is to account for the fact that lead-acid batteries are only 85-90 percent efficient (as stated by Stephen A. Pain in the September Motorhome magazine article).? That means, as he goes on to say, "that if you consume 1000 watts from a battery, it might take 1200 watts to fully recharge it."? I'm guessing that it also factors in the fact that a battery's capacity is always overrated by the manufacturer and slowly decreases as the battery gets older.

As far as the charger needing a constant current output for the posted equation to work, I believe the battery charger amperage factor used in the equation is the maximum current used by the charger.? A 10 amp charger puts out a maximum of 10 amps but may put out less as the battery is charged.

Ned misstated the 12V capacity of his four 6V AGM batteries above.? I believe he meant to say that each are rated at 110 AH and not 220 AH.? Actually, they are rated at 220 AH in a 6V application, but only at 110 AH in a 12V application.? To turn 4 6V batteries into 12V, each pair of the 6V batteries have to be connected in series and then the two pairs have to be connected together in parallel.? I like the fact that AGMs will take all the amperage you can throw at them as long as you keep the voltage below something like 14.2V.? To me, recharging four batteries from 50% SOC to 80% SOC in two hours or less is phenomenal.? However, I don't like the expense of updating to 4 AGMs or the cost of a 100 amp smart charger.? Even the author of the Battery Basics article said "In many installations where the batteries are set in an area where fumes or leakage are not a concern, flooded batteries remain a good economic choice." 

The point about float charging batteries after they are 100% charged has nothing to do with the posted equation, but it is something that should be considered and could also have been discussed in the article.? Desulfation is another item that is very misunderstood by most RVers.? I'd like to see more information on that subject myself.? Maybe Motorhome magazine will come out with another article soon going BEYOND Battery Basics.

 

[Tom]

Ned misstated the 12V capacity of his four 6V AGM batteries above.  I believe he meant to say that each are rated at 110 AH and not 220 AH.  Actually, they are rated at 220 AH in a 6V application, but only at 110 AH in a 12V application.

Rolf,

Ned quoted it correctly. The 6V golf cart style batteries have a capacity of 220AH. 

To turn 4 6V batteries into 12V, each pair of the 6V batteries have to be connected in series and then the two pairs have to be connected together in parallel.

That's how his (and mine and many others) are connected. When you connect two of those 220AH batteries in series, the capacity is still 220AH, but the voltage is 12V instead of 6V. Connecting a second pair in parallel the way you described provides a total capacity of 440AH at 12V.

FWIW I use 10 of those batteries on my boat. Each series-connected pair has a capacity of 220AH at 12V and the 5 pairs connected in parallel gives me a total capacity of (5 x 220 =) 1100 AH at 12V.

 

[Tom]

Even the author of the Battery Basics article said "In many installations where the batteries are set in an area where fumes or leakage are not a concern, flooded batteries remain a good economic choice."

There's nothing "wrong" with flooded cell batteries and I've used them in this application for many years. As the author says, they're a good economic choice. It just means you have to check the fluid level periodically.

Desulfation is another item that is very misunderstood by most RVers.  I'd like to see more information on that subject myself.

You're in luck. What you're describing is the need for periodic "equalization" of wet cell batteries to remove sulphates that may have built up. Essentially, an equalization cycle overcharges the batteries at a higher than normal voltage while maintaining a fixed current. The resulting boiling action removes the sulphates. It's discussed a little more in our library article here.  *

 

[Ned]

As Tom described, I have 4 of the 220AH 6V AGM batteries in a series/parallel configuration for a total of 440AH@12V.  It doesn't matter how the batteries are configured, the AH rating does not change.

 

[rsalhus]

Ned quoted it correctly. The 6V golf cart style batteries have a capacity of 220AH.

You're correct Tom, Ned did quote it correctly.? I misunderstood it when I read it the first time.? Sorry Ned, I think I'm just jealous of your AGMs and your converter/charger system.? Some guys just have it all.? ;D

 

[Tom]

...Ned, I think I'm just jealous of your AGMs

Me too Rolf. Ned just has more money than the rest of us.

 

[Ned]

Never did, and certainly don't now   Tom, you have more AGM batteries than I'll ever have.

 

[rsalhus]

Ned,
On the subject of batteries, I see from your blog that your 12V disconnect switch went out recently.  You apparently removed the switch and went merrily on your way.  I'm impressed.  Was that hard to do?  Got any pictures of that for those of us that might run into the same problem?  I'm assuming the 3 large cables are from the generator, shore power, and the 12V batteries, correct?  Could you walk us through the steps you took to remove the switch and how you reconnected the cables?  I'm sure others would be interested as well.  BTW, nice website and great photos!