Sane battery management plan ?

[Jones]

Hi from sunny England,

I run a system that's in daily use in a camper/RV truck.

It supplies varied loads like boiling kettles, powering tools as well as charging phones, laptops, with heat and lighting duties too.

Daily usage takes our bank down to about 85 percent SOC. 

Am really happy with the system and it's capacity but, had not given much thought to a proper battery management plan until today.

To maximise the service life of the batteries I'd really like to only start the solar charging cycle when we drop to around 70 percent capacity, rather than whenever the sun is out.
   

Am I right in thinking that this is going to be near impossible via the rebulk voltage setting with such a range of loads?

I noticed an older post that it may be possible "in the future" via the WizBang.

-Would really appreciate some advice if anyone can offer it!

[Rain Dog]

Am I understanding correctly that when your batteries drop to 70% you want to automatically charge the batteries from a generator or other charging source?

[Jones]

Hi,

Thanks for replying, I've altered my first post to explain what wed like to achieve a little better.

There is no other charging source.  Just solar.

I'd like to stop the bank charging every single day. 

Hoping I can use a setting on The Kid, but am also open to using a switch or another hardware solution if needs be.

[Westbranch]

A daily cycling to  ~ 75 -80% should not be hard on your batteries as you seem to have the capacity to recharge to 90+% daily, am I correct?

What you want should be possible with a WhizBang jr if you have the matching to your auto  start on your generator...not sure about your Victron...

Halfcrazy would be the one to tell you...

we cross posted, but yes the WBjr can be used to stop charging.....but WHY do you want to stop the CC from doing its job?

[Rain Dog]

Assuming you have lead-acid batteries, your plan will shorten your battery life. However if you want to do it anyway, you can certainly do it by adding a battery monitor to your system such as: http://www.victronenergy.com/panel-systems-remote-monitoring/bmv-600s-and-bmv-602s and using the programmable relay.

[Vic]

I'd like to stop the bank charging every single day.

Hoping I can use a setting on The Kid, but am also open to using a switch or another hardware solution if needs be.

Hi Jones,

Perhaps you are looking for the '  Skip Days  ' function that exists in the Classic CCs?

Believe that this is not yet available in the KID (might be wrong).   This works very well,  and use it here on both of the larger battery banks.

As long as you have fairly large battery Capacity vs loads,  and have a reasonable amount of recharge power,  as in PVs,  this should work well.

Just my guess,   Vic

[Jones]

Well, its quite possible that I've completely misunderstood the concept of charging cycles!

Is it not correct then that the less cycles a bank completes in a given period, the better?

My thinking was to use some more of the existing capacity by chaging less often.

Cheers for the replies so far chaps.

[Rain Dog]

The deeper you discharge, the less cycles you will get out of the battery.

http://www.solar-electric.com/deep-cycle-battery-faq.html#Cycles vs Life

Using the lifeline example from the link: assuming you get to 100% each cycle, if you go from 100% to 85%, you should get about 3500 cycles. If you got from 100% to 70%, you should get about half that, so basically it is a wash.

[Westbranch]

and shallow discharge (<10%)  can be just as bad as too deep a discharge

[Vic]

Jones,

What type of battery are you using -- Flooded ?...   AGM ?   ...   Gel?

What is the AH Capacity of the battery bank?

What is the total Label rating of the PVs that recharge the battery?

AGMs have a real DOD vs Cycle Life,  much more so than Flooded batteries.  Generally,  AGMs will withstand about 50% fewer cycles to a given DOD than Flooded Lead Acid (FLA)  batteries,  at least for discharges below about 85% SOC.

As Wb mentioned,  very shallow cycles,  ie cycling to 90 % SOC,  or even less deep than that can damage Flooded batteries,  at the very least.  This is very true.

Cycling Flooded batteries below 80% SOC on occasion,  is often good for them.   Even deeper discharges to 50,  or 60% SOC can be even better for FLAs,  but perhaps only a few times per year.

SO,  for the larger FLA banks here,  skip 3 - 4 days with essentially NO CHARGE,  by setting Vfloat low -- 2.10 V per cell,  50.6 for a 48 V bank.   This keeps the CC Float voltage from recharging about 50% of the previous day's discharge.

Opinions will vary on this approach,  but this way can get down to about 75 - 80% SOC,  depending on the A/C loads,  and that is the goal.  These two battery banks for two different systems are each ten years old,  and knock  ..  knock  seem to be doing well.

Opinions.  YMMV,    Vic

[Jones]

Yep understood.


Rolls have some published charts showing cycle life vs depth of discharge for their flooded and AGM "renewable" battery range.

Vic, I use two of their 12v 240AH capacity AGM batteries in series attached to 400w of PV.

According to the info sheets, these batts have a maximum expected lifespan of 1300 cycles. 

At 50 percent DOD, they rather surprisingly say that this will drop to 1200 cycles. Other figures given are 700 at 80 percent (and 500 for 100 percent DOD..)

My question has to be then: 
With a finite amount of available cycles, does it not make sense to preserve these by discharging more deeply it possible, rather than burn one each day with a shallow discharge of 15 percent?


If this is the case and I haven't just misread the material, a skip day function would be pretty useful. 

Even better I guess, would be some way that I could set the SOC reading from  WhizBang to retard or trigger a solar charge cycle from The Kid.

Is this thinking unreasonable?


Thanks for your help here guys.

Jones

[Vic]

Hi Jones,

Have not yet looked at the Surrette AGM data on cycle depth vs number of cycles.

But believe that a curve that one could draw from that data might favor not cycling much below 85%,  on average,  if one could  --  will look for the data.

Believe that the primary limit of DOD vs number of cycles is that the catalytic recombiners  in the upper cell area (essentially the "caps")  wear out with the number of cycles.   Essentially (IMO)  the more time that the battery spends gassing (this is mostly during Absorb),  the more wear on these elements which re-combine the Hydrogen and Oxygen that is liberated from the water in the electrolyte during Gassing.   There may also be a heat built-up factor,  such that long absorbs associated with deep discharges may wear on these re-combiners in an accelerated fashion ...   I cannot say for certain.

It is possible,  that for AGM batteries avoiding cycles much below 80%,  on average,  might shorten battery life excessively ...  some of the consideration is how many hours in the Absorb stage are required to fully charge the batteries,  and if deeper average cycles remove more life from the battery.   It may not just be the consideration of total AH delivered to the loads,  as the wear on the AGMs may not linear,  if you know what I am trying to say.

FWIW,   Vic

[Jones]

Vic, thanks for the good advice and describing the science.

Rolls supplied a more detailed graph today (attached) showing max life on these AGMs is an estimated 2750 cycles at 20% DOD.   


A few more years in them than originally thought then. Bonus.

Would definitely still be interested if The Kid is able to stall a solar chaging cycle somehow. 

If the UK gets lucky and this good weather continues it'd be a useful function.

Will flip input braker tomorrow as SOC still over 90% from earlier float.

[Vic]

Hi Jones.

Thanks for posting the Surrette chart,  showing DOD vs Cycle Life.  Those look like a pretty good battery.

I did look on the Surrette site,  but could find none of these charts for AGM batteries.

Thanks,  Good Luck,    Vic