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Old 21-09-2018, 17:49   #1
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Undercharging batteries on solar with Victron MPPT

For 2 months now I've been working on some undercharging issues with our 2 lead acid 100Ah house batteries. They're only 2 years old and were showing signs of reduced capacity. The batteries are Interstate brand, group 27.

Now I figured out we were probably undercharging occasionally, but am hoping some more experienced people can check my interpretation and solve some remaining mysteries.

Our primary charging source is 200W of solar. A bit under-solar'd (planning to add more) but we're a relatively low electrical usage boat (10-20 Ah/day + 10-60Ah fridge use, which we vary based on how much solar we have and how the batteries are doing - never go below 12.2, and usually not below 12.3).

We've been cruising for 6 months and have shore power 2-3x/month (Xantrex TrueCharge 20). We don't motor enough to consider that anything more than a bonus bulk charge source. The rest of the year we're on shore power all the time except for the occasional weekend winter cruise.

Last week we left the boat for 10 days on shore power + solar (we had to fly to a wedding) and when we came back I found our Link 10 showed +35.5 Ah. When we left the boat it had been fully charged with 24 hours of shore power, showing about +6 Ah on Link 10. (+6-8 Ah after a shore charge or good solar charge is pretty typical for us - the Link 10 auto resets the baseline periodically). I've never seen it go anywhere near +35 before though!

The shore charger was in float 13.7 the whole time, so I think this means our solar absorption cycle of 1 hour of 14.8 each day pushed 30 additional Ah into the bank (that's 16-20% of our 150-180Ah bank! I'm assuming our 2-year-old batteries don't have the idealized 200Ah anymore).

So it seems like we have what Mainesail calls "floatulation" - both our shore charger and solar controller are going into float before the batteries are really at 100%. But the Victron MPPT solar controller is a pretty good one. For us it typically does only 1 or 2 hours of absorbtion before dropping to float, based on the day's starting voltage. I bumped up the absorb voltage to 14.8 and float to 13.7 after first suspecting undercharging (note these are true voltages measured at the battery - I compensated +0.1 after measuring 0.1 voltage loss between the controller and batteries). We have 55F water here so the batteries are usually cool.

90% of days we complete the solar absorption cycle, including the day prior to the 10-day period where we somehow pushed an additional +30 Ah (with no usage). At the end of absorption the batteries are typically accepting < 1.0A.

So why is the Victron dropping to float when the batteries can accept another 30 Ah? There's no way to force it to do longer absorption - it drops to float based on voltage, charge acceptance, and morning startup voltage. Perhaps I should use its auto equalization feature every 2 weeks?

I equalized the batteries on shore power 2 months ago but I'm not sure how effective that was. At start with a full shore charge, cells ranged 1.230-1.250 (1.265 is target) and on equalization our shore charger does 15.5 for 6 hours and then times out. I had to run it for about 16-18 hours (is this really long for equalization??) and all cells got to about 1.245-1.250 but none to 1.265. Cells were bubbling but none needed topping up. Amp acceptance started at 2.3a and gradually dropped to 1.4a (0.7a per battery) by end.

I'm guessing the fact they didn't reach 1.265 means they've some capacity due to undercharging about 1 time per month in the last 2 years? This was surprising to me though - I thought deep cycle batteries should last about 5 years, and undercharging is probably a lot more common than our 1 out of 20 days.

So I think my questions are:
- Is my understanding right and is there anything else I can test?
- If Victron MPPT set to 14.8/13.7 and reaching float on most days is still undercharging our batteries, what can I do to get it to stop doing that?
- Did I do equalization correctly? Is it common to not be able to get the batteries to 1.265 after 18 hours?
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Old 21-09-2018, 18:12   #2
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Re: Undercharging batteries on solar with Victron MPPT

Interstate does not make quality batts suitable for deep cycling, pretty sure made by JCI, meh.

As far as the current State of Health (reserve capacity) of the bank goes, if they have been chronically PSOC cycled, a couple of years could easily be EoL by now.

Only way to really tell is a 20-hour load test, unless you recorded a quicker standard benchmark when they were new to compare to.

Like even the best adjustable charge source, that Victron controller's actual Hold Absorb Time must be adjusted - usually lengthened from the default - until the bank gets to 100% Full* at Absorb V at least a few times per week.

Unless Interstate publishes a more specific endAmps spec, that means waiting until trailing amps declines to around 1A for those 200AH

before the voltage is reduced to Float.

Best case with a bank in good shape* depleted by say 30-50%, that takes 4-6 hours even when charging amps are at the max CAR* (plus concurrent loads carried*)

With a worn bank, it may take so long, use the Defa FLA spec for 100% Full, hold Absorb V until "Current change over 1 hour period of less than 0.1A" instead.

Now this is when cycling. Sitting at Float is another matter.

If the charge current is always high enough to fully support concurrent loads, then the bank should be held at 100% Full by the Float charge.

* ask if you don't fully understand
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Old 21-09-2018, 18:27   #3
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Re: Undercharging batteries on solar with Victron MPPT

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Originally Posted by Tessellate View Post
For us it typically does only 1 or 2 hours of absorbtion before dropping to float, based on the day's starting voltage.
I presume only after many hours of Bulk, striving to reach Absorb V.

What is the pattern when on shore power, say 60+A available?

> I bumped up the absorb voltage to 14.8 and float to 13.7 after first suspecting undercharging

What was Absorb before?

> At the end of absorption the batteries are typically accepting < 1.0A

With what instrumentation do you get that measurement?

> There's no way to force it to do longer absorption

Are you using the dongle / SmartSolar menu? I thought there was.

> Perhaps I should use its auto equalization feature every 2 weeks?

Yes but not auto, do it manually, ideally on shore power.



> If Victron MPPT set to 14.8/13.7 and reaching float on most days is still undercharging our batteries, what can I do to get it to stop doing that?

Set Float to Absorb V when cycling. Set it back when stored no loads pulling SoC down.

> Did I do equalization correctly? Is it common to not be able to get the batteries to 1.265 after 18 hours?

Interstate should provide the protocol.

I agree your bank is likely at EoL, but they should be cycle tested on shore power.

Your charging infrastructure should be tested with a new bank.

The best battery value by far is Duracell (actually Deka/East Penn) FLA deep cycle golf cart batteries, 2x6V, around $200 per 200+AH pair from BatteriesPlus or Sam's Club

Trojan T-105 may be worth spending a bit more.

12V batteries as good can be harder to find.

Consider getting a clamp ammeter if you don't have one.

Link 10 is a bit long in the tooth, possibly inaccurate

I like Victron 712-BMV if you need the AH counting.

Merlin / Balmar SmartGauge is more accurate for SoC and easier to maintain, but does not show amps nor AH.
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Old 21-09-2018, 19:24   #4
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Re: Undercharging batteries on solar with Victron MPPT

A charger in float is still putting a couple amps into the bank. So everyday the ah will go more plus. Until you unplug. It will reset to 0 as soon as you start drawing. I've seen banks at like +250ah left all winter on charger.
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Old 22-09-2018, 06:52   #5
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Re: Undercharging batteries on solar with Victron MPPT

If the charger AHT is adjusted properly for the context of your use patterns,

so that it usually only drops to Float after endAmps is reached,

AKA you've eliminated the premature infloatulation problem,

then no further charging will take place until AH start to be removed from the bank.

Which will not happen, as long as the Float charge's current is sufficient to carry any loads present.

Victron's Absorb Return algorithm (I believe not adjustable?) is tuned to kick in much more frequently than many other SCs, IMO a good thing for bank longevity.
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Old 22-09-2018, 07:21   #6
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Re: Undercharging batteries on solar with Victron MPPT

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A charger in float is still putting a couple amps into the bank. So everyday the ah will go more plus. Until you unplug. It will reset to 0 as soon as you start drawing. I've seen banks at like +250ah left all winter on charger.
+1

The batteries will accept a small current even when they are fully charged and on float. The battery monitor is just counting the AHrs and assuming, incorrectly, the batteries are becoming more charged.

35.5 AHrs over 10 days means on float the batteries were accepting 0.15A (assuming no house loads and that the battery monitor did not reset itself while you were away) which is normal and does not indicate the batteries were not fully charged when you left. If you have any house loads, such as an occasional bilge pump, or even small indicator LEDs the batteries have accepted even less current.
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Old 22-09-2018, 07:35   #7
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Re: Undercharging batteries on solar with Victron MPPT

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Originally Posted by smac999 View Post
A charger in float is still putting a couple amps into the bank. So everyday the ah will go more plus. Until you unplug. It will reset to 0 as soon as you start drawing. I've seen banks at like +250ah left all winter on charger.
Not trying to hijack the thread, but this got me thinking...

My boat sits on a mooring and has 90w of solar (2 x 45w panels in parallel) and an Outback PWM controller for a 220a 12v battery bank. I never see more than 13.4v on the battery terminals when charging. When I leave the boat the battery is at about 75% SOC (12.4v). It sits for weeks between uses. Is my battery getting back to 100% SOC over these weeks on the mooring?
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Old 22-09-2018, 08:26   #8
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Re: Undercharging batteries on solar with Victron MPPT

The only way to tell is to measure trailing amps at the mfg Absorb spec voltage.

If it is higher than their endAmps spec, then the bank is not yet fully charged.

If you don't know that spec, use .005C, half an amp per 100AH.

Or if the bank is badly worn and never seems to get there, use the Deka version, hold Absorb V until

"Current change over 1 hour period of less than 0.1A" instead.
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Old 22-09-2018, 08:52   #9
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Re: Undercharging batteries on solar with Victron MPPT

We found the default absorb time on the Victron controllers was to low. Its very common for users to bump that value to 6-8 hours using the bluetooth dongle.

We also found that the absorb voltage of 14.8V was too low for our bank as the losses between the controllers and batteries meant the Victrons were measuring 0.1-0.2V higher than at the battery terminals. We bumped our absorb voltage to 15.0V and now see 14.8 at the battery terminals during the absorb phase.
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Old 22-09-2018, 09:05   #10
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Re: Undercharging batteries on solar with Victron MPPT

I wonder if some small load is not going through the shunt properly and so is not being captured. 12.2 is 50% discharge for me. If I’m discharging overnight on a daily basis, I try to stay above 12.7 so the batteries will last longer. My VictronConnect controller and shore power charger seem to agree on battery charging and state. They are very much in sync, once I set the bulk and float voltages to be basically the same.
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Old 22-09-2018, 09:17   #11
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Re: Undercharging batteries on solar with Victron MPPT

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We found the default absorb time on the Victron controllers was too low.
True for every charge source I've encountered.

Adjustable gear is not meant to be left at the default settings.
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Old 22-09-2018, 09:31   #12
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Re: Undercharging batteries on solar with Victron MPPT

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We found the default absorb time on the Victron controllers was to low. Its very common for users to bump that value to 6-8 hours using the bluetooth dongle.
I think the factory default is 6hrs.

This is not the real absorption time which can be anywhere from 0 to 6 hrs on this setting, but might typically be about 30mins to 1 hour of genuine absorption time, although the controller will always report this as between 1 and 6 hours.

Yes it very confusing

Changing the 6hr default setting to 8hrs is only a very slight difference. This will only typically give another 5-10mins of genuine absorption time, so work in larger steps at first, but look at large number if cycles as the genuine absorption time will be very different for each cycle.
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Old 22-09-2018, 09:49   #13
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Re: Undercharging batteries on solar with Victron MPPT

Most controllers define AHT as

How many hours after the setpoint is achieved (Bulk is complete) until I drop to Float?

I understand that high-current loads can stop charging, even deplete the bank enough that the whole cycle should be restarted,

and a controller would ideally only count actual time at Absorb in their AHT algorithm.

But what SCs actually do that?

Obviously actually controlling the stop charging point based on endAmps via a shunt at the bank is the only **truly** accurate method,

but even the dumbest egg-timer unit can usually get "good enough" results as long as they are adjustable and usage patterns are pretty consistent.
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Old 22-09-2018, 09:53   #14
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Re: Undercharging batteries on solar with Victron MPPT

You could raise the float charge to 14.4 or so
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Old 22-09-2018, 09:54   #15
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Re: Undercharging batteries on solar with Victron MPPT

Found noelex77's past thread exploring the issue wrt Victron specifically

Victron MPPT Battery Charging Algorithm
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