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Old 10-18-2020, 06:37 AM   #1
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Simple battery confusion

I bought a Victron BM712 battery monitor now im confused..



my system is simple.. i have 2 regular group 31 Batteries for starter batteries..



I have 2 Batteries-plus brand group 31 AGM batteries as House batteries.


each pair of batteries is wired parallel.. each pair of batteries creates a 12 volt "pack"



I have a battery isolator between the 2 packs of batteries..


there is 15 feet of 2/0 welding wire between the 2 packs of bateries



the bus alternator has a 10 ft 4 gauge wire that goes from it to the starter-battery pack.


batteries-plus gives me VERY LITTLE info about my AGM batteries.. other than they are multi purpose.. car,boat,camper, starting, house.. just that they are 100AH each so 200 for the "pack"



when I run the bus or connect a battery charger to the house battery pack of course they charge up.. but everyone tells me AGM batteries are supposed to stand anywhere from 13.2 to 14.1 when fully charged.. I cant get that info from Batteries-plus.. they are clueless.. I do know that I use power and the batteries dont "die" quickly.. however if I charge them wit ha battery charger overnight. then let them sit for a couple hours they always go back to just about exactly 12.6 to 12.7, I disconnected the isolator to make sure it wasnt engaging the starter batteries and running down my AGMs.. yet I never no matter what I do get them to stand any higher..



is it possible these AGM batteries are designed to stand at 12.6-12.7? ir has 4 years of alternator charging just conditioned them to sit at that voltage?


ive pulled 60 or 70 amp hours off of them over a few hours time and they still are holding oiut at 12 volts or slightly above. (im told 10.5 volts is when they are considered "dead" 0%) ive only ever gotten them close to that a couple times when I parked the bus and acidentilly left. the inverter on which turns off at 10.5, I didnt have the victron then so I dont have a gauge of how much it took to charge them.. although in those cases I connect a 15 amp automatic battery charger and let it slow-charge them back..



how do i know what to fill in for all the fields in the victron when the manufacturer has no info? the batteries still seem "good" so I dont want to get new ones just to get new ones..



do i charge em up with a battery charger then do a steady amp load like say pull 20-30 amps of them all day and let them go "dead" and see how many AH were pulled off as a test?



are there better batteries to use for my purpose?



im not using any permanent solar.. I do have a portable fold-out solar panel with its own charger (set for AGM) that ive played with before.. in ideal conditions I think that is designed to be a 200 watt panel.. but its not permanent just a toy really..



my bus is mainly a driver so alternator is its main means of charging. since i do park it at the "barn" at night when im at home i can charge it with a battery charger.. (I have access lugs so the BM712 will monitor charging amps to the pack via battery charger, toy solar, or alternator)..




-Christopher
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Old 10-18-2020, 08:22 AM   #2
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Hi Chris,

12.7 volt is charged and sitting for a couple of hours.. slightly depends on temperature. .Agm in general has a lower charge voltage.. the equalization with flooded lead acid is higher because you can easily blow of the excess energy in h2 , S and boiling. That is normally around 14.4 volt... Vehicle alternators can go up to 14.4 volt.

I would think an old fashioned diode insulator would work good in matching your flooded starter and Agm house together.

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Old 10-18-2020, 12:24 PM   #3
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yeah my alternator charge is usually at about 13.9 - 14.1, the bosch alternators do a nice job of regulating right in that range..



so would i use the diode in place of the isolator I currently have? Just set it up so current would flow from the start side to the house side but not back?



what keeps me from running down my starter batteries in that case?
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Old 10-18-2020, 12:59 PM   #4
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The "internet" says AGM full resting is 12.7, and that is what you see, so doesn't seem to be any problem with your batteries. AGMs are great, seem to have a shorter life as I recall, but tolerate full discharge better, which is what you want for a house battery.

Battery isolators are just diodes that only let current INTO the house battery.

I have no idea what a Victron BM712 is or does so can't help you there. :shrug
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Old 10-18-2020, 02:04 PM   #5
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my isolator is a solenoid that kicks in and out.. there is a little "computer" that detects when the alternator is charging or not.. or if the override switch is on. and engages / disengages the solenoid..


the BM712. is a battery monitor.. measures the Amp-hours going in and out of the batteries and also attempts to show you a state-of-charge..



good to know that 12.7 is a good number..



Ived been known to over-think things a time or two
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Old 10-18-2020, 03:03 PM   #6
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Quote:
Originally Posted by BeNimble View Post
AGMs are great, seem to have a shorter life as I recall, but tolerate full discharge better, which is what you want for a house battery.
No, going much below 50% drastically shortens the lifespan of any lead bank, to be done rarely in emergencies only.

The BMV is a proper shunt-based Battery Monitor, does a pretty decent job estimating SoC% when installed and used properly.

Which is not a trivial learning curve, and IMO should be taken out of the circuit while basic troubleshooting and design issues are being resolved.

Unless that is the only ammeter available, trailing amps is how you tell if your bank is getting fully charged.

http://marinehowto.com has excellent articles on these topics, well worth parsing in detail.

Including 2-3 relevant to the Battery Monitor topic.
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Old 10-18-2020, 05:35 PM   #7
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I have optima batteries and standing voltage should be 13 according to them, and I do find that to be true if fully charged. I do also find my alternator does not bring them to full charge. Standing voltage after sitting overnight if I have driven it till the end of the day will be about 12.7. The solar panels top them off the rest of the way the next day.
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Old 10-18-2020, 05:56 PM   #8
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I have optima batteries and standing voltage should be 13 according to them, and I do find that to be true if fully charged. I do also find my alternator does not bring them to full charge. Standing voltage after sitting overnight if I have driven it till the end of the day will be about 12.7. The solar panels top them off the rest of the way the next day.

I do notice that my portable solar panel charges with a higher voltage than the bus alternator or the automtive battery charger does.. maybe I have to find some type of proper charger?



https://www.amazon.com/Renogy-Batter...731225313&th=1


ha! or maybe I'll just keep doing what im doing because i havent destroyed my batteries in 4 years.... but this is the first fall / upcoming winter. that the bus will get heavy use. of the parking heat system.. I might get a real AGM AC charger since i do park it nxt to an outlet most nights.. I can charge proper overnight
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Old 10-18-2020, 06:18 PM   #9
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Yes a full charge takes 7+ hours for lead, no matter how many amps are available.

Of course if amps are too **low** then much longer is required.

Quality deep cycling AGM - in NA Odyssey, Lifeline or Northstar are the main makers - recommends at least a 0.2C rate for longevity, but 0.4C is better.

Deep cycling FLA is cheaper, easier to care for and lasts longer, but can't accept more than a 0.2C so no point paying for more.
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Old 10-18-2020, 06:33 PM   #10
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what do you mean by 0.2C, 0.4C etc?



so for topping off after my driving day maybe i need something like this onboard when I plug the bus in?



https://www.victronenergy.com/charge...roof#downloads



the alternator of course does a partial charge and it seems if ive run them down to 11.9 or 12.0. it hits the pack pretty hard at 50-60 amps and then backs off.. if its a relatively short drive home say 20 minutes or so. i'll notice them charging less amps lioke maybe 20-25.. if its a long drive home.. ill see them at say 5 or 6 amps.. thats off the alternator.. the alternator voltage seems to lower a bit as the bus heats up.. someone once told me thats on purpose that as the alternator gets warmer it assumes ther batteries are also warmer and lowers the output voltage. .. no idea if its true. but i do notice its a few tenths lower when hot than when cold..



-Christopher
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Old 10-18-2020, 07:01 PM   #11
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Quote:
Originally Posted by cadillackid View Post
what do you mean by 0.2C, 0.4C etc?
-Christopher
C rate refers to the ratio between battery bank capacity and current.

"1C" = 1:1 ratio
For example if you have a:
100Ah battery and charge at 100A your C-rate is 1C.
100Ah battery and charge at 200A your C-rate is 2C.

100Ah battery and charge at 5A your C-rate is 0.05C (also sometimes represented as C/20)

Another relevant aspect of this is how C rate relates to time.

1C = 1 hour until 100% fully charged or discharged
2C = 1/2 hour until 100% fully charged or discharged
0.5C = 2 hours until fully charged or discharged
0.05C = 20 hours until fully charged or discharged
*by fully charged or discharged I mean from 0-100% or 100-0%





C-rate can refer to charge or discharge. Another term to be aware of is "fractional C" which just means <1C
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Old 10-19-2020, 05:17 AM   #12
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I would use the Victron or one similar that is plugged in after driving
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Old 10-19-2020, 06:31 AM   #13
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Originally Posted by dzl_ View Post
C rate refers to the ratio between battery bank capacity and current.

"1C" = 1:1 ratio
For example if you have a:
100Ah battery and charge at 100A your C-rate is 1C.
100Ah battery and charge at 200A your C-rate is 2C.

100Ah battery and charge at 5A your C-rate is 0.05C (also sometimes represented as C/20)

Another relevant aspect of this is how C rate relates to time.

1C = 1 hour until 100% fully charged or discharged
2C = 1/2 hour until 100% fully charged or discharged
0.5C = 2 hours until fully charged or discharged
0.05C = 20 hours until fully charged or discharged
*by fully charged or discharged I mean from 0-100% or 100-0%





C-rate can refer to charge or discharge. Another term to be aware of is "fractional C" which just means <1C

cool stuff! so a good battery charger would be programmable and work to maintain the 'C' which you set forthe battery type?



or is the ideal that the charger knows the published charge voltage for the battery (if such a thing?) and when the battery reaches that voltage the charge current essentially drops to 0?
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Old 10-19-2020, 07:34 AM   #14
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I always thought that with any lead battery, you charge to a certain voltage and then the amps should taper off as the battery nears full, where you would then switch over to a float charge.

The chargers can monitor your amp usage and convert that to a battery capacity %, but I don't think that's a definitive number. Once the battery is float charged and is full, that's it. It doesn't matter if your 100AH battery has 90AH of capacity or 120, once it's full, it's full.

Likewise, once the discharge voltage gets to a certain level, you can consider the battery to be at 0%, regardless of whether that was 100AH of power used or not. There are various trains of thought on low voltage cut offs, some say 11.6 is the magic number for low voltage cut off, others feel it's closer to 10.5V.
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Old 10-19-2020, 09:00 AM   #15
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I always thought that with any lead battery, you charge to a certain voltage and then the amps should taper off as the battery nears full, where you would then switch over to a float charge.

The chargers can monitor your amp usage and convert that to a battery capacity %, but I don't think that's a definitive number. Once the battery is float charged and is full, that's it. It doesn't matter if your 100AH battery has 90AH of capacity or 120, once it's full, it's full.

Likewise, once the discharge voltage gets to a certain level, you can consider the battery to be at 0%, regardless of whether that was 100AH of power used or not. There are various trains of thought on low voltage cut offs, some say 11.6 is the magic number for low voltage cut off, others feel it's closer to 10.5V.

my inverter has a programmable shut off.. they recommended 10.5 volts as the shut off for house batteries. and 11.5 if using starting batteries.. (apparently most vehicles will still start at 11.5).. my diesel heater has a cut-off point of 10.5, so im using 10.5 as my "we are dead" point. simply because all of my devices start dying then which renders the battery useless at that point..



im going to get a top-off charger that i can plug in at night along with my block heater.. when im on a road trip chances are the fact im driving for usually 6 hours or more will likely give me a decent charge.. of course I cant really plug in on a road trip.. though if I do park someplace during the day I could set up my portable solar panel.. (most often I travel to reach sunny warm places and away from cold cloudy!)
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Old 10-19-2020, 09:17 AM   #16
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Whatever you use is up to you. I don't have solar or a battery bank on my bus, so I'm not all that up on charge controllers and the devices that are being discussed. I do have some experience with maintaining lead batteries, though. My figures came from kendrick instruments. A while ago I was looking for a cheap cigar lighter voltmeter, and then stumbled across their article here.

https://www.kendrickastro.com/lvc.html

What they said made sense to me based off of what I've observed in longevity of fork lift batteries that I've maintained. Those that charge nightly, typically have longer lasting packs then those that run them near dead and then recharge.
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Old 10-19-2020, 09:34 AM   #17
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that articale makes a sense of it more.. so essentially i need to reprogram my thinking of what a "dead" battery is..



my brain thinks of a dead battery as one I put a meter on and get a big ole 0.. in essence a dead battery is one i put a meter on and get a 10.5. thus a standing voltage of 11.5 is essentially a battery at 50% charge.. if I want to maintain the longevity of my batteries i dont want to cycle them below 50% unless i absolutely have to..



thus in my case since im not trying to be an off-gridder, when I notice my voltages getting below that say 11.8 or so as im habging out in my bus, its time to start the engine and pump some juice back in.. and at night I plug em in when available as to start out with a nice full battery each day...



in my mind deep cycle meant that I could constsently run em to 10.5, but sounds like doing that results in a destroyed battery sooner than later.. much easier just to turn the key and fire up the bus every so often as needed..
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Old 10-19-2020, 09:49 AM   #18
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Do some googling, read up on the fundamentals.

Voltage setpoint is ideally adjustable, or at least dip switches for the various chemistries you might buy in future.

Amps rate is an "offered" maximum, the bank takes what it can, depends on chemistry and SoC.

So if your bank is 400Ah and you want 0.4C, that's 160A, higher than most, but 0.2C at 80A is easy.

Actually being able to reduce the max amps offered is a lithium thing and rare.

In early Bulk/CC stage, low SoC, amps are high, as voltage rises.

When CV is reached (say 14.4V) that is Absorb stage, amps are declining, many hours remaining if the Bulk rate was high.

When Amps fall to 0.005C that's how **you** know the bank is 100% Full. (endAmps spec varies by make/model)

If you don't get there most cycles, longevity suffers.

Ideally you can adjust Absorb Hold Time on the charge source, 99% drop to Float way too early.
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Old 10-19-2020, 09:56 AM   #19
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If you camp off grid, just using FF sources is not practical for maintaining your bank properly

OK for high-amps Bulk for a couple hours in the morning, but you really should use solar for the 4-6hrs remaining required to get to Full.

Using your vehicle alternator while stationary is silly, get a little genset.

Or overnight on shore power, at least a couple nights a week.

Or just replace the bank more frequently.

Or invest in LFP.

Yes, any lead bank allowed to drop regularly below 50% is being abused, just as much as not quickly getting back to 100% Full.
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Old 10-19-2020, 12:51 PM   #20
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When Amps fall to 0.005C that's how **you** know the bank is 100% Full. (endAmps spec varies by make/model)
Thanks for the spec. I was always curious as to when they were and finding them online seems to be difficult or inconclusive.

Quote:
Originally Posted by john61ct View Post
Yes, any lead bank allowed to drop regularly below 50% is being abused, just as much as not quickly getting back to 100% Full.
What's the end result of not getting back to full quickly? Is that when sulfation occurs? Does the specific gravity of the acid drop permanently?

On the forklift/man lift/ golf cart batteries I always determined full SOC based off the specific gravity of the acid. Once it was into the green area of my hydrometer(around 1260+ IIRC), and they were within 50 cell-cell, I assumed the battery was at 100% SOC.

Are there DC-DC charge controllers out there so that one could charge a battery back up to full in the correct stages, vs charging and holding at the 14.5 volts the alternator creates?

I have an onboard diesel generator with a 60 amp alternator. I'm hoping to install a house battery bank of 3 group 31 deep cycle batteries, and use that to power a DC fridge and LED lights. My plan is to have no solar, and have battery charging accomplished by running the generator. The fridge won't be on all the time, only when camping, and in that scenario the generator will likely be running anyways to power the air conditioner. Would I be fine charging the bank with the alternator, or should I find a charge controller.

Sorry Christopher for the hijack, but I've kind of had the same questions and thoughts that you have regarding a house bank.
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