Pb charging: absorption finishing amps and state of health

tracking  endAmps

The Absorption stage of lead-chemistry battery charging is characterized by constant voltage (Vabs) and decreasing current.  The battery takes less and less current as it approaches full charge, and finally plateaus at a low rate.  Current is the red line in this graphic:


That leveled off terminal rate in the right third is called endAmps, finishing amps, tail current, etc. It is typically C/200 to C/100, or 0.5 to 1A per 100Ah of battery capacity.  Fresh batteries tend to have lower endAmps reached more quickly, and tired / old / abused batteries tend to have higher endAmps reached more slowly (longer Absorption phase required).

 > I have repeated this test with AGM batteries [in] worse shape that exceeded 7 hours to attain 100% SOC, likely due to sulfation.

If that is so, one might be able to record endAmps monthly as a metric for battery state of health.   It might not tell you when a particular battery is toast, but increasing endAmps would be an indication of design, all other things being equal.

MainSail suggests endAmps greater than the 0.75%C (C/133) indicates the battery is no longer running at full capacity:

Over many years of capacity testing hundreds and hundreds of batteries, flooded, GEL and AGM I have yet to see a lead acid battery that could [deliver] its full storage potential at anything less than 0.75% in tail current at absorption voltage.

The wording is a bit odd there, and IMO means “worse (higher) tail current than 0.75% rather than ” < 0.75%”.

increasing Vabs when Absorption duration is limited

Charge controllers have different strategies for knowing when to end Absorption

  • high end controllers may hold Absorption until the specificed endAmps is achieved as long as it takes (or as long as sunlight is available).
  • mid-range controllers may hold Absorption for a duration specified by the user
  • low-end controllers usually hold Absorption for a set duration like 2 hours.

In each of these cases, batteries in poor health may require longer than the available time to drop to endAmps.  One workaround for maxxed Absorption duration is to increase Vabs.  I propose increasing Vabs in 0.1v – 0.2v steps until endAmps can be achieved within the available duration.

Caution:  as Vabs creeps up you will either hit the maximum allowable setpoint (AGM, Gel) or need to water more often (flooded).


longshot:  decreasing charge rate when Absorption duration is limited

Charge time remains relatively constant whether charging at max or min rates for the battery.  Again quoting Mainsail:

charging AGM at C/2.5 (max rate) resulted in the battery being 63.3% “full”3) at Vabs and being fully charged in 5hrs, 30 minutes (20min Bulk, 5hr 10mins Absorption)

charging AGM at C/5 (min rate) resulted in the battery being 77.4% “full” at Vabs and being fully charged in 5hrs, 42 minutes (76min Bulk, 4hr 26mins Absorption)

It is conceivable that charging at a minimum rate (particularly for flooded that can be successfully charged as low as C/10) could also help absorption finish faster.  Overall time would be about the same but the charging process would spend more time in Bulk which by definition is not time-limited.


My own set of flooded golf cart batts (2x 6v in series) is Absorped at 14.6v @ 70F and finishes at 1.79A, or 0.81% (C/123) in about 2.5 hours. This suggests I have lost some capacity in 589 cycles at 50% DoD (installed 9/24/2018).  I wasn’t measuring it before so I can’t compare to new, and the loss of  capacity is not noticeable in practical experience.

When my bank can no longer reach endAmps within the three hour max the controller allows I will try increasing Vabs.


backchannel: “my [200w] solar refills my batteries by 10:30 am nearly every day!”

I think it’s worth talking about so onlooker solar newbies don’t get confused about the abilties of their solar config, worry about whether or not it’s working, and/or murder their lead batteries.

From this post on the Promaster forum.  This is the system in question.  The author and I are running the same battery bank.  He charges it with 200w of panel and I charge with 570w;  I still don’t get my bank fully charged “by 10:30am nearly every day”.   It can happen (a night of shallow cycling, early AM driving, etc)  but it’s not normal.   I am not calling BS;  rather, I suggest critical information is either being left out or not understood.   Either way it’s bad for solar newbie onlookers.


I think 200w of panel is a real sweet spot for vandwellers. I like that amount and recommend it as a sane default.

> my solar refills my batteries by 10:30 am nearly every day!

Depends on what “refilll” means.  It’s pretty easy to hit (an incorrectly measured) “100% SoC” by voltage ~12.8v or even Vfloat.  It’s pretty tough to get deep-cycled Pb fully charged (ie, completed Absorption down to battery manufacturer spec — endAmps) with solar-only charging by 10:30am.

Given that lead-chemistry batteries take 2-3 hours to finish Absorption after deep cycling, Vabs, usually in the mid-14v range, would have to be attained by 7:30am-8:30am. Not too many solar-only systems can do that, even significantly overpaneled ones charged by MPPT under excellent ouput conditions (temperature and insolation).

Caveat to onlookers: the controller switching to Float does not mean that Absorption stage was successfully completed. It means the controller switched to Float.  The way to know for sure that Absorption is being completed is to watch your meters during late Absorption and verify that the battery is accepting something like a typical  C/200-C/100 cutoff (or whatever your battery mfg recommends). For a typical 200Ah lead bank this would be something like 1-2A. If your controller allows you to set endAmps of C/200 or whatever, go ahead and set it. Most consumer controllers don’t do this (or can’t separate out battery charging and loads to measure accurately) so you may have to just max out any Absorption duration setting that is available. Once you know your system is finishing Absorption correctly you can let it fly on autopilot. If you are offgrid and your controller refuses to finish Absorption, you might choose the nuclear option and set Vfloat == Vabs (check water levels more often).

Example with data: I am putting ~175w into a 220Ah bank this partially-cloudy morning in NV and at 10:30am local I still hadn’t kissed my 14.6v Vabs. This puts me at a fully charged (“filled”, IMO) between noon and 1pm. My system has more panel but because I am running loads on my system (~200w worth of laptop and crypto mining) this is a pretty good approximation of how a 200w:215Ah system with no loads would charge the bank on a good day.

If I stop being an idiot and don’t run the crypto rig until later in the day I can typically get fully charged by noon assuming my preferred level of snowbird insolation. 🙂 This is with an overpaneled system and not the usual experience with solar.

Having said all that, banks cycled lightly to ~80% or assisted by alternator (or other) charging in the early morning would have a decent chance of being fully charged by 10:30am.

backchannel: solar is a luxury

I generally don’t look at the username or other details of a poster’s presence when replying;  I stick to the “content of their [text] character[s]”, to paraphrase MLKjr.  This can cause me to waste time casting pearls before known swine.

So I didn’t realize that the post was by a daydreaming pothead who frequently bombs half-baked ideas at us in /r/vandwellers.  This particular one didn’t seem as moronic as usual (only confused and overconfident) so I engaged.  It had the title:  “How often do you drive your van? I think solar is a luxury. I’m thinking driving to work and to buy food ect is going to be plenty of time for the alternator to recharge 4 deep cycle batteries. Instead of having solar you could recharge with a system your van already has.

So here’s what I wrote before I realized s/he probably wouldn’t even remember writing it later:

Solar charging and alternator charging are different beasts that work better together than either do separately.

Solar isn’t mandatory if you can plug in a smart converter at work a couple times a week to get the bank fully charged.

People who want to charge from alternator only would be wise to look at lithium or carbon-foam chemistries which don’t care about partial stage of charge (the scenario you presented). Lead batteries die early from partial charging.







backchannel: solar harvest, battery charging

from this post:

Although solar is sold to the public as watt/hr rated panels, to achieve their sales rating the panels must be oriented perfectly facing the sun, on a cloudless day, with no obscurations or diffusing humidity to achieve their rated power.

I’d say it’s an apples-apples objective rating which salescritters might [mis]use to make a sale. For onlookers, lab ratings are done at 1000W of light per square meter with cell temps of 25C (ambient approx 0C.)

Cell temp is specified because panel maximum output decreases as cell temperature increases. Luckily it can be calculated easily. Note to new folks: losses due to heat are the reason we leave airspace under panels. That way air can get under the panels and wick away heat from the bottom of the cells, which is typically 50-60F higher than ambient.

Example: today’s 100F temp in El Paso means my panels will lose 21.24% Continue reading “backchannel: solar harvest, battery charging”

backchannel: Lifeline batts and alternator charging

I follow the on Meryl and Me Hit the Road blog.  I read an article there today and thought some of the tech details would be worth talking about.  The article discusses the charging system[s] for the RoadTrek’s new house battery bank.

the isolator

[the battery seperator] is a “smart” switch – when the engine is running and the RV batteries need to be charged it connects the engine battery to the RV batteries and charges them through the van’s alternator. It also will charge the engine battery when the RV is plugged into shore power

This sounds like a dual voltage sensing relay (VSR).  Such a relay “senses” when either side has power to share and then connects the starter and house batteries.   The VSR is powered by the starter battery and is always “on” watching the voltage on both sides.

[A normal isolator (constant-duty solenoid) on the other hand gets power from the ignition wire so it is only “on” when the key is turned on. ]

Let’s unpack the quoted paragraph a bit; there’s a lot going on. Continue reading “backchannel: Lifeline batts and alternator charging”

CTEK + Battle Born?

from a reddit post:


[Battle Born] are the defacto standard for drop-in LFP. And they have a tremendous warranty.

The value of the CTEK D250SA is that it can DC-DC boost alternator voltage to lead acid absorption voltages (Vabs), which are usually higher than one needs to charge LiFePO4.

Internally the lithium pack is fully charged around 13.8V. BB recommends pushing at 14.4v, probably because it would speed up charging a bit. The BMS will keep the pack from going over their actual full charge point.

Note that speeding up charging (ie, increasing current) is not necessarily what your alternator wants. Lithium already slurps up current at fierce rates.

BB recommends a “float” of <=13.6v. Lots of folks use 13.2v-13.4v Vfloat to keep the bank from resting at full.

I have not seen the details of the CTEK’s gel and other profiles. It is possible that one of the profiles will fit well with BB’s suggestions. If it does, you’d still be maxxed at 20A unless you want to add a $$$ smartpass.

You may observe that 13.4- 13.8v is a pretty normal range for alternator output, and the alt is unlikely to go over the 14.4v BB recommends for charging. If I were charging a BB with alt-only (no solar) I’d be tempted to use a plain solenoid to charge at alternator voltage. No $$$ DC-DC charging needed.

Note that the BMS can reject charging suddenly, and when the solenoid kicks in it will start charging suddenly. I have heard of folks wiring in a small AGM battery (garden tractor, motorcycle size) to act as a buffer to protect the alternator. It might even be ok for it to be a dying battery, dunno.

Both the CTEK and the BB are spendy gear. Do read other opinions and do more homework before investing. As far as you know I could be a lunatic in an asylum who has sneaked in a smartphone. 🙂

backchannel: single battery systems

from this post:

Can a portable solar system be used to charge your existing vehicle battery and then continue using the vehicle as your primary source of power? If that is possible it might eliminate the need for secondary battery (weight and space).

Yes.  I describe that usage in the Shallow Cycling rvwiki article.  Trebor and Sternwake on CRVL have both done it successfully.

I’m guessing that might shorten the life of traditional vehicle battery, IDK…..

We don’t know.

My reading suggests that starter batteries average about 36 months of useful life.  The main failure modes are: Continue reading “backchannel: single battery systems”

backchannel: CC controlling amps?

from this post:

The charge controller isn’t allowing any further amps into the batteries.


It’s happening, but the controller isn’t doing it.  The batteries aren’t allowing any further amps into the batteries.

The mppt controller reacts to decreased current acceptance by moving Vpanel toward Voc; in the first display we can see Vpanel as 39.8v.

When the system is loaded (second pic) the MPPT algo has picked one of at least two points at which it could get 5.6A at 13.2v for the battery.  We know it can’t be Vmp or the panels would put out too much.

This pp happened to be below Vmp, at 17.3v.  One would presume another one would be above Vmp somewhere.




“proper input charge voltage”

from this thread:

I didn’t see this mentioned but a “12v battery “ is technical in a deep state of discharge at 12.1-2 volts.

The reason you didn’t see it mentioned is we weren’t talking about that.

The proper input charge voltage should be 13.8 volts from your solar panels/ charge controller. A fully charged 12 v battery should have a nominal voltage in the range of 13.5 volts.


Are you making this stuff up?  It is ok to misinformed.  It is not ok to state misinformation as fact.

For newbies:  Vabs and Vfloat should be set to whatever the battery manufacturer recommends.  Full charge on FLA battery is measured with a hydrometer, and generally correlates with something like 12.7 – 12.8v.