How often should AGM batteries float to keep them in good shape?

Hello,

I'm in my first off grid Canadian winter. Happily on the Pacific Ocean...so not too cold. But the sun sure is low in the sky and the trees to the south of the cabin sure are tall. I've discovered that there is never unimpeded sun on the two 260 watt solar panels.  The sun moves across the horizon in November letting only dappled light hit the panels. Clearly I'm going to have to move them to a sunnier location and perhaps purchase more panels too.  In the meantime, I'm noticing on the Outback 60 charge controller that the system never hits absorb or float. It did up until October...but not since then. If I use my computer all week (heavy use of batteries), the output drops to 12.0V. I have four 6-v AGM batteries set up as a 12-v system. I've come to learn from googling that I shouldn't let the Rolls S6-275 batteries drop below 12-v too often or I shorten their life. Do you agree with this statement?

I've been using a generator and battery charger once a week to recharge them.  That generally brings the batteries up to 12.5v and slowly over a week of use they drop to 12.1. The charge controller does say MPT Bulk a lot so there is some charging happening...it just never has enough time to float.  

What is a good general practice? How often should AGM batteries be floated...through solar or generator, to take good care of the batteries?

Thanks so much,
Julia

Batteries like to be full. If they were in float every day would be the best scenario.

For generator charging i would run the charge until the display says absorb and shut the generator off to let the solar panels finish the charge. Best use of fuel and generator service life. If it has been more than a week with low sun availability, then go ahead and run the generator until the battery hits float.

At 500ah your battery could use a little more pv watts, especially since you have shade issues.

Its been similar here with great solar exposure, just so cloudy for the last month and a half.

Usually i will size pv watts based on the season of worst sun availability as long as it is practical buget-wise and integration-wise.

So when you look at consumption, assume 5%-10% output from the array, or worse. 850ish watts is about as much as you want to allow as far as charging current is translated. Luckily, your controller can limit charge current to 65 amps as needed. The extra watts would only come into play in cloudy or shaded operation. I would imagine you get only 20w if that under the condition you are describing, but luckily, your controller has pv watts displayed along with watt/hours and a daily log going back 180 days.

Nice components. It does not look like you use much power in a week so you should be able to use the live display and the logs to determine how much power you use. A battery monitor is helpful, but you should be able to extrapolate what is going on within the system under actual conditions with just the mx60 display features.

If you cant move it to perfect exposure, supersize it!

What does the log reveal about kwh/day and what do you see as pv watts under the conditions you described?

I agree, get your batteries up to float every few days, even if some energy austerity is involved, because what would you rather spend money on, replacing dead batteries frequently, or buying more panels to keep your batteries charged and expand your energy busget long term.  

You might consider angling the panels to near horizontal if your mounting allows.  This does 3 things for use winter maritime people particular at high latitudes
1)  shade on any part of your panels (wired in series) brings the whole performance down nearly to the light intensity of that shade.  No amount of brightness on half a panel will overcome shade on the other half.  In winter, with the sun low in the horizon, shadows are hard to avoid unless your panels face up.
2) cloudy and overcast days, theres not much sun to track and getting a full day of diffuse light can net you better than having a 2 hr window of direct sun through a lot of atmosphere.  The ideal seasonal angle calculators are all based on clear winter skies. Even if you had no shadow problems, the weather alone might make that trade better.
3) do you get strong winter winds,  near horozontal can reduce the physical strain.... though angle it enough to slough snow and drain rain.

I keep one string of 3 panels,at 56 deg, and one string at 27 degrees in winter (adjust 4x a,year for,seasonal,angle) on clear days the 27deg panels do better, i get marine fog in mornings and many overcast days and the 56deg string does better.  

Hi Frank,

Thanks for responding! I have a samlex SEC-1230UL battery charger. It has an ammeter (display) with a needle that starts out at 15 DC amperes when I first turn on the generator and over about 6 hours moves to 0. That's when I turn it off - as per the manual. But the outback charge controller doesn't reflect the generator charging the batteries. If I start charging the batteries in the dark at 7am the charge controller will remaining on snoozing. And 6 hours later when the charge controller is at 0 the charge controller may be showing bulk if there is enough light or it might still show snoozing if it is raining. So the charge controller never shows absorbing or floating when I use the genny to charge the batteries. So I can't be sure the batteries are floating even with a charge. But I'm still thinking the batteries probably are floating if I'm charging for 6 hours and the ammeter shows 0 at the end. Do you agree?

I didn't use any power except a DC light all weekend. So practically no load. I charged the batteries with the genny on a sunny Saturday and as Sunday was sunny too I was curious to see on Sun eve if the charge controller had reached float as the batteries were at 12.5 and had not been used all day. Nope. Hadn't even hit absorb. So 'topping up' with solar doesn't seem to be an option in the current location in November.

I will go look through the logs and note the kwh/day and pv watts and report back. I'm not 100% sure I have the settings on the charge controller optimized. I don't understand what all the elements mean or everything you said.  But I'll note them down and get your input!

Joseph - What an interesting idea to put the panels more horizontal to get diffuse but unshaded light. I could try that for a week and see what happens. Once I learn to read the logs better!

Thank you both,

Julia

Julia Watson wrote:Hi Frank
I was curious to see on Sun eve if the charge controller had reached float as the batteries were at 12.5 and had not been used all day. Nope. Hadn't even hit absorb. So 'topping up' with solar doesn't seem to be an option in the current location in November.

I will go look through the logs and note the kwh/day and pv watts and report back. I'm not 100% sure I have the settings on the charge controller optimized. I don't understand what all the elements mean or everything you said.  But I'll note them down and get your input!

Julia

Rolls states charge parameters as 14.84v absorb and 13.94 float (68 deg. F./20 deg. C.) And i would place end amps 00 and absorb time at 5 hours to start, until you find out how many hours an absorbtion charge takes. General rule is 1 hour per 100ah capacity. This could require more or less time depending on how you have re-bulk set and how deeply you take the batteries on average. I have found the 1 hour/100ah to be a good place to start.

I would use the remote temp sensor (if you dont already) and program the appropriate t-comp setpoint for the rolls agm.

The settings can be critical, and after 7 years of installing as an occupation, i am still tweaking the way i program and learning more every season! Owner feedback and maintenance services i provide to several systems teach more every visit, no matter how much i read.

If you want to see the display active while charging with the generator, you can try to force bulk or force float in "misc" settings. The battery voltages from the generator charger operation will be seen on the display and you are looking for the indication at the setpoints above. So, it should hit 14.84 (absorb) for 4-5 hours and then go to 13.94 (float)

A 15a charger will surely take all day and then some to complete a charge 16-20 hours! Thats alot of fuel and wear.

An optimum generator charger should be between 50 and 60 amps. This way the bulk charge is done pronto and absorb will take less time also, compared to 15a rate. The idea is also to load the generator at 50-75% of max continuous load, so you have room for running more than the charger and dont throw away a bunch of fuel where the charger at full stuff only uses 10 or 20% of power at idle.

This is common, people dont want to load a generator "too heavy" so they run a 7kw machine and supply power to a refrigerator and some lights. This is folly and expensive in noise and chemical pollution, material and fuel.
Load heavy during run time and leave 20% max overhead unless there is a real reason to "loaf" the generator.


The last 25% charge to float will take as long as the first 75%, generally. Its the reason to try to run to absorb or maybe absorb for an hour or two and then top up with solar (shade and length of day permitting).

As far as understanding goes, i cannot stress enough to read the manual several times. I do it like a binging junkie, first i go to the interesting stuff, then i go back and read the stuff that supports the interesting stuff and then i read and re-read front to back..... its hard not to jump right to "fire in the wire", but everytime i have been caught being uninformed or mistaken or broke down and called tech support, it has been something overlooked in the manual. Some of the very best manuals are written for renewable energy equipment, but painfully  comprehensive they are also, and it takes time.

Its like reading fiction novels, especially fantasy or sci fi, at first it is very confusing to me, as places and names are foreign and need to be learned.
The manuals (especially the Gudgels manuals, trace, zantrex, outback and now midnite solar! These are the designers of most all the heroic power electronics in the arsenal over the years!) Are carefully laid out with explanation of all operations and nomenclature.

Technical language is the most rapidly evolving of any dialect and the reference to function needs to be understood in a word or phrase or it would be many volumes to get through the introductions. This is the usual stumbling point people are discouraged by, i find.

If you start at the begining of the manual and with your already sound ability to observe and operate the equipment as you have, it will come as natural after a few.

Some settings in the outback controls are alittle esoteric, like setpoints for wake or sleep in mpp or voc%, park, end amps, etc.the main thing is getting it to respond to low light and that the voltage, max amps, re-bulk and time in absorb is correct. Then you can tweak on it. It takes a couple weeks if the weather will not co-operate, maybe a winter if you are not home to monitor often during good conditions.

If you post photos of your dashboard at various conditions and post the setting you have as they are it would be educational for others as much as yourself as you sort out the settings. Ill take a look and other people here will likely have stuff to add.

12.5 volts from an agm battery after a "full" charge with no load is a bit low and i would look for 13.1 or so after completing a cycle and at rest.

12.5v at no load would indicate 80% state of charge. This will wear out a battery in a season or two easily, especially if at 12v or lower, often and for protracted periods of time. Which is not your case, but if the battery lives going not higher than 80%soc, it will not last nearly as long as if it is discharged more deeply but charged full often.

I read most of your charger manual. (Not several times cover to cover...... but its yours, not mine!
30a not a bad charger and samlex products have alwaws performed admirably for me.

Dip 1 and 2 should be set to off (but try s1 off and s2 on if you have very low standby loads and it may actually help you achieve higher voltages, but is warned against in the manual for good reason.... needs to be monitored especially with sealed batteries, i would avoid if you are not confident or turn off all loads while charging) and it looks like both absorb and float voltages are a bit low for your battery. It means when the charger says "done" the battery wants (needs) more. More time at higher voltage or at least more time, which it will not do. Still a great auxilliary charger for you, but this is my main issue with chargers that do not feature programable setpoints.

Hello Frank,

Thanks for all that info! I'll comment on some of it:

-Rest assured I read the manuals. Sometimes several times. But that doesn't mean I understand it all! It's a process.
-I generally set the charger DIP Switch to S1 Off and S2 On and make sure I don't have anything turned on in the cabin. I'm thinking that speeds things up rather than using a load and setting the DIP switches to off/off.
-I do charge everything I can from the second outlet on the genny whenever I run it all day. I have an extension cord inside that leads to the inverter that everything plugs into.
-I have been adjusting the charge controller settings depending on the average temperature. It is on average 10C currently so using the Rolls reference chart I have Absorbing set to 15V and Float set to 14.1V. I may get a automatic temperature sensor at some point if that proves to be key.
- Now that you've read my battery chargers specs (very impressive!) and know that it is a 30amp charger, do you think 6 hours could be about right for it to float?
-When charging I have been putting the Honda EU2000i generator in Econo mode. It uses less gas and is quieter. But will that only increase the amount of time needed to charge the batteries?

My charge controller settings are currently:
Absorb End Amps = 20A (I see you recommend 0A can you say a bit more about what changing this setting will do?)
Rebulk Volate = 12V
Snooze Mode = .6A
Wakeup 1.5V, 5 mins
Park MMP 77% VOC

I wrote down the logs for the last 32 days and then entered them into a google sheet.  If you are up for looking at them and commenting on what they reveal I'd be appreciative. I've taken a copy and left it in edit mode in case it makes sense for you (or anyone) to comment on cells in the sheet. https://docs.google.com/spreadsheets/d/1nYhw40QSh-Rc2kFR1kSwqsBKPfDpcUxmK3U-6fos1Kc/edit?usp=sharing

Thank you!

Julia

End amps is best used with a pair or triple shunts, flexnet dc and a mate controller. It terminates the charge when the setpoint would exceed absorb volts. 00 amps is off and resorts to absorb time. If you have the accessories, the system will compensate for incoming charge from other sources like your generator charger and the loads while using end amps as a target. It cannot see outgoing power and will keep charging at the load rate plus end amps value before it floats, possibly harming the battery.

Eco is the right mode as the charger only draws 400ish watts max even if it drew more id still use eco mode it will not restrict power output, it will allow the generator to waste way less fuel at low demand.

Your end amps setting if used would be 10a., no load. I would bump up rebulk to 12.2-12.4 and see how it acts.

As you are now, all charging sources combined would not exceed max charge amps for the battery, but ideally you would have two ac chargers to the or 60a capability and less generator run time.

You would avoid allowing any combined input beyond max allowable charge amps of the battery. 65-80 amps is about as much current as they can handle. Refer to the maker.

An inverter charger with a transfer switch (nice because it can be a 75 - 100a programable charger) or another charger (2) with an accessory transfer switch would allow you to keep everything plugged in and simply pull start your generator without using extension cords.

I use absorb time as a limit, but i also use specific gravity to find it. agm does not allow this

Your logs show an average of 100-150 watt/hours per day solar charging.  And the generator and pv combined did hit 15v.

The thing i would look at first aside how often it floats, and how much power comes in and lowest battery voltage,  is from a full battery, how many watt/hours to full after a whole day of use without solar input and with solar at average use.

When your battery is in float, you can the see how much power you are using. You must subtract the floating watts from the value and it will be actual consumption of loads aside from float charging at that time.

A better explaination of end amps function is when the addition of charge current required to exceed absorb voltage is less than the end amps setpoint, the charge is terminated. If you add a load or other charging source, the controller cannot tell the difference without reference to the current across the shunt(s).

I am not sure about seeing only 15 amps from your 30a charger, it reacts to battery voltage, but every battery your size and larger, i have seen will soak up more amps if available, so it could be just not high enough voltage for your cold batteries.

Battery temp plays a huge role in determining charge voltage and i would use the remote temp sensor. I have tried mine without and the results are impressive. My margin of safety would not get the batteries where they needed to be reliably with 20degf swings in battery box temp.

I am suprised that setups without temp comp do as well as they do. Most controls have an ambient temp sensor, some do not. If batteries are in the same area as the controller it will be close, but i greatly prefer not pushing buttons all the time to try to stay on top of a function that is easily automated and made correct and reliable for $20!

You could connect a shop charger with more amps to the pv input of a controller and get what you need. The charger you have is nice, but it seems it will not allow high enough voltage for reliable and fast charging.

.2 volts is alot, .6v or .8v is more than alot of difference. Samlex may have a way to increase absorb voltage.

More current. Unless the sun is nearly full on, you simply do not have enough charging current available for 500 ah. The battery requires 25a minimum at proper absorb voltage to charge the battery in a serious manner. Twice that is the spot.

On its face (or name-plate), 30a would do, especially with good sun. But the panel reading of 15a is not sufficient. The charger will likely only do 30 amp if the battery is at 10.5v. It has enough power if the battery is near full, but thats not its purpose.

300w is 300w (charger) and 1000w available to your battery through-out bulk and early absorb is appropriate while suiting generator size.

You may have to turn the pv breaker or controller to off, in order to get your charger to play nice. Its either that or set your fm60 to the samlex spec. and try that.