Battery charging info

[Roads_Less_Trav]

After living on 12 volts full-time for 8 years, I've put together a series of blog posts sharing everything I've learned about battery charging, different kinds of charging systems and the unique quirks of solar charge controllers.

We've installed a variety of systems and lived with equipment from many manufacturers, and this is all the info -- in plain language -- that I wish I'd had access to when we were designing and installing our systems and scratching our heads trying to understand how everything really worked once it was installed.

I'm not pretending to be an expert, but we've spent over 2,500 nights living off of battery power and have learned a lot in the process. In the hopes that I might help out other RVers, I thought I'd post the links here.

RV (& Marine) Battery Charging Basics
- Single vs. multi-stage charging, in depth

Converters, Inverter/Chargers, Engine Alternators
- How "artificially powered" charging systems work

Solar Charge Controllers
- How to optimize battery charging from a "naturally powered" charging system

I hope this info proves helpful to others...

[Roads_Less_Trav]

Byrogie -

I have posted some info about what happens when solar charging is combined with battery charging via electric hookups or a generator or an engine alternator... I hope it is helpful:

RV Battery Charging - Solar Power and Shore/Alternator Power Combined

[time2roll]

Duty cycle may depend a bit on battery size and usage.

400 AH would be fully charged as current approaches 2 amps. But you cannot set it for 5% (45 x 5% = 2.25) because usage may keep you in absorb all day.

I would try something like 50% duty cycle and 15 minute timer. This should give a short time in storage and extended time when in use. Adjust the duty cycle if absorption seems too short or long while in use.

I think there is a help function in MSView to make suggestions. Not sure if the duty cycle % applies to your solar amps or the controller rated amps.

[ewarnerusa]

Morningstar Tristar PWM 45 uses time at or below a duty cycle percentage at abs voltage as a float trigger when you use the MSVIEW software to customize the charging profile. The user selects both the duty cycle percentage and the time at or below that duty cycle for when the controller switches from absorption to float. Does anyone have any advice on those parameters? Thanks.

[time2roll]

MS MPPT-60 is not trying to drop to float at exactly 100% charged as much as just making some adjustment for Storage vs Normal use vs Severe usage.

Storage; a short absorption is fine.
Normal; usually needs a couple hours. (mine is programmed to extend at 12.5)
Severe; holds absorption voltage all day. (mine is programmed to cancel float at 11.9)

Bogart or Victron that ties into a battery monitor will have a better shot at dropping to float right at 100% charged.

[jrnymn7]

Yes, I've been considering this Time in Abs thing, and my gut was telling me it had to do with when amps tapered... based on C-rate. Indeed, if comparing a 10% rate to a 20% rate, on my 430Ah bank, it would take about 1 hour to do an 80-90% charge at a 10% rate, and about 40 minutes at a 20% rate tapering. From 90-100% soc, they would be equal.

So, these controllers that adjust Abs Time must be looking at beginning Vbatt (which is not that reliable, seeing as the controller has no idea if the bank is under load, or how big a load), and considering how long it took to get voltage up to the Vabs set-point. Obviously, a 20% rate would do that quicker than a 10% rate. So, a 50-80%, (+129Ah's), at a 20% rate (86a), would take about 1 hour 30 min., while a 50-90%, (+172Ah's), at a 10% rate, (43a), about 4 hours. And that extra 2 1/2 hours in 'bulk' would correspond to another 20 minutes in Abs.

So, yes, I see now, how that could be done, but with limited accuracy.

[BFL13]

The ugly graph says you get to a higher SOC with a lower charging rate, but once tapering starts you are on the same slope till done.
Compare the constant vs tapering times in the three sets.

[jrnymn7]

"If they were more discharged at the start of the charge cycle, they will need to spend more time in Absorb."

In that case, they would need to spend more time in bulk/cc... Abs/cv would not differ. Charge Rate and Vabs setpoint decide how long Abs will take, not dod beyond the transition from bulk to Abs.

Real World vs. Marketing

Like I've said, a 50-100% charge at a 10.5% C-rate will cause amps to taper (i.e; go into abs) at 89.5% soc, and a 70-100% charge will likewise cause amps to taper (i.e; go into abs) at 89.5% soc. The numbers don't lie. The depth of discharge beyond the point of tapering is irrelevant, regardless of what any marketing strategy would suggest.

[Roads_Less_Trav]

jynrnn7 - The Outbacks use the Bulk time to calculate the Absorb time because the length of time the batteries spend in Bulk is largely dependent on how discharged they are. If they were more discharged at the start of the charge cycle, they will need to spend more time in Absorb.

Morningstar handles this by looking at what the lowest voltage was that the batteries reached overnight. If they got below a certain level, they stay in Absorb longer, and if they got to a really low level they don't go into Float at all...

The only limitation to that approach is that if they got really discharged overnight but some other charging system got going before the sun came up, they might be fully charged at sunrise and not need to spend much time in Absorb.

This would happen if you had a big alternator on your MH or boat engine and you started driving/motoring at midnight and went all night... by sunrise you wouldn't need much solar charging if any.

[BFL13]

Can't link to it now but ISTR it was the Deltran (sp?) site with the graphs of its charger profile. It explained it all there. Was some years ago I last looked at it. Had one graph for Wets and another for AGMs except they were the same ISTR.

[jrnymn7]

According to the examples I gave above, it would take 4X that 1.9 hour bulk charge to get from ~ 89% to full, and 2X that 3.8 hour bulk charge. This, again, proves chargers with preset parameters do not have what it takes to get the job done, under anything other than 'preset' (predetermined) dod's.

In this particular case, I would be forced to do 50% dod's, just to get the bank fully charged using that 2X time factor... thus decreasing battery life, either by chronic undercharging, or deep discharging.

No predetermined 'factory formula' will do.

[BFL13]

The Battery Tender or one of those does have a profile that relates time in Absorption to the time it spent in Bulk. ISTR A time is twice whatever B time came to.

Absorption can go on forever, so some kind of cut-off is needed so it will start Float. Many chargers use about 97% SOC as their cut-off point to drop to Float (such as the PD converters) and the typical "automatic" charger will shut itself off there too.

That is where all this " 2% acceptance rate means full" comes from that I hate so much, when to me, "full" means 100% not 97%. 😞

[jrnymn7]

re; SWITCHING FROM ABSORB TO FLOAT BASED ON TIME

I fail to see how time in bulk is relevant to time in absorb? At a given voltage, and a given current limit, amps will begin to taper when the bank is at a particular soc. This will not change, whether the the bank was at 50% soc or 70% soc to begin with..

The point at which bulk/cc switches to abs/cv is determined largely by charge rate and Vabs setpoint. How long it takes to get to this point in no way affects how long it will take to get from this point to full.

I have my parameters set such that amps will taper when % C-Rate = % dod, and Amps to bank = Ah's depleted. So, a 45a charge on my 430Ah bank (a 10.5% C-Rate) will result in amps tapering at 10.5% dod (-45Ah's). But the time spent in bulk/cc, before this occurs, does not effect this. A 50-89.5% bulk charge will take about 3.8 hours, while a 70-89.5% bulk charge will take about 1.9 hours. But either way, amps will begin to taper at 89.5% soc/10.5% dod, and will take the same time from there to 100% full, regardless.

[Roads_Less_Trav]

Byrogie - That info is coming in the next post, as we've experienced it all, including running an engine alternator to boot. These articles take a lot out of me to write, so I'll be taking a break for 10 days or so before I write that last post in the series. In a nutshell, nothing bad happens, but depending on how things are set up, one system becomes dominant, and you want to be sure the other systems don't throw up their hands and quit, if you need them to keep working. Thanks for reading!

[Roads_Less_Trav]

Chandalen - Thanks for your input. I'm learning about all the software options from the many posters here. Very cool. Thanks everyone!!

pnichols - We actually upgraded to AGM batteries on our trailer in April and had them for all our years on our boat as well, so I discuss them at length in the first post in the series on battery charging (you probably didn't check out that post since you've been around that block already... no problem!). I really appreciate all your detailed notes on AGMs and will add the tidbits I missed! Thanks for reading my other goodies in the past too!!