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Backup/Secondary Battery Management


Jaco De Jongh
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I have a secondary 12 volt System running, maintained through a 75/15 MPPT, charging straight from my 48Volt bank. 

I prefer keeping it at 100%  at all times as the 12Volt system supplies all the security, communications and monitoring equipment in my house. 

I use 2 x 100Ah Discover Mixtech EFB (Deep Cycle) batteries and even being in the RV/marine class imo it does claim to have quite a few cycles at 30% DOD using the Mixtech technology. 

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My question, after reading a lot of opinion from a lot of different sources.  What would be the best way to manage battery's like this. 

I dont want to discharge them on a daily basis, but also dont want to keep them at 100% for ever. I have read that its good to cycle batteries from time to time, but with no real guideline as to how often and to what DOD?

For two nights in a row now I pulled the fuse feeding the MPPT at the same time the PV production on the main system stopped and pushed back the fuse when PV production started again the next morning. Both nights the Normal consumption took it down to 90% SOC. (Measured with a 702 installed on the Secondary system) 

 

Any recommendations or experience to share here?

 

 

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8 minutes ago, Jaco de Jongh said:

For two nights in a row now I pulled the fuse feeding the MPPT at the same time the PV production on the main system stopped and pushed back the fuse when PV production started again the next morning. Both nights the Normal consumption took it down to 90% SOC.

That is a lot of current for that little 75/15. Impossible. Maybe the bigger bank is siphoning something out via the MPPT, as panels have diodes to stop that?

80% DOD is supposed to be the nirvana for deep cycles.
Maybe 90% for sealed maintenance.
The pics seems like 30% DOD for 1400 cycles ... do that for if you don't use them they are going to die of old age, guaranteed.

Me, every couple of months I go for SOC of 50%, and I keep them at SOC of 80% the rest of the time.
Make sure they are at least once a week at 100%, if there is rain falling and all that. ;-) 

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10 minutes ago, The Terrible Triplett said:

That is a lot of current for that little 75/15. Impossible. 

2.5 to 3 Amps take it down to 90% in roughly 13 Hours  (Or that is what the 702 is reporting) When I push the Fuse back it draws about 14 amps recharging the battery's, I have limited it on 14Amps. 

Forgot to say, Only the charging current goes through the MPPT, when I pull the fuse the load is drawn straight from the Battery's, And with the fuse in the MPPT will do the 14Amp and as the Battery fills up  the amps decrease till it reaches the 2.5A normal running load.  

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2 minutes ago, Jaco de Jongh said:

2.5 to 3 Amps take it down to 90% in roughly 13 Hours 

What I meant to say, if that little 75/15 was on panels, at night, it draws mili amps at best.

What is causing the draw, via the 75/15, that is the question.

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4 minutes ago, The Terrible Triplett said:

What is causing the draw, via the 75/15, that is the question.

All the 12Volt equipment connected to this system. (Normally its a bit higher, but a couple of PIR's ext isnt connected due to some renovation and relocation of the Equipment) Normal load is about 4Amp constant.  

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I would let if be at 90% after 13 hours, means you can go +-26 hours to reach +-80%.

They claim 1400 cycles at SOC of 70%, not so? That is nearly 4 years.

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While I have you on the line, Max panels for the 75/15, I know you know it off hand so I don't have to google?

If I decide to do that I might as well split the 2 systems, run them independently. I have 4 x 150 Watts just sitting in the garage not doing anything. 

220Watt max on a 12 Volt system, Can that be correct? 

What would happen if I connect 300Watt to it?

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50 minutes ago, The Terrible Triplett said:

That is a lot of current for that little 75/15. Impossible.

By my own tests it can run at 14.3A for hours on end (it derates a little when it gets hot). 10% DoD is 20Ah. You can have that back into the battery by lunch.

I do like the fact that you can tie the Victron controllers to a battery and use it like a buck converter. There are some cheaper controllers that cannot do that.

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33 minutes ago, Jaco de Jongh said:

While I have you on the line, Max panels for the 75/15, I know you know it off hand so I don't have to google?

Have 1 controller per 150w panel, onto the same bank, all set identically.
Or 2 panels per controller ... :-)

image.thumb.png.0a8988c938705eb6958ff1c2820ee56c.png

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5 minutes ago, The Terrible Triplett said:

Have 1 controller per 150w panel, onto the same bank, all set identically.

Eina, do you still have the 100/30 up for sale? Think i saw something in the classifieds recently? 

 

Sorry forget this question, not really worth it, just checked the specs.. 

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7 minutes ago, plonkster said:

By my own tests it can run at 14.3A for hours on end ...

In the "impossible" context ... 20mA used when panels are off, the I recalled, Jaco does it in style - 48v bank being the "panels", so it is nothing ito current draw by that little gem.

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Just now, Jaco de Jongh said:

Eina, do you still have the 100/30 up for sale? Think i saw something in the classifieds recently? 

Nope, keeping it, the 100/30 and 150/35 are and will continue to jointly supply power.

75/15 can take 220w, so add 2 x 150w panels per controller.
At 25% oversize that gets you to 275w, controller can handle the rest. 

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2 minutes ago, The Terrible Triplett said:

At 25% oversize that gets you to 275w, controller can handle the rest. 

@Jaco de Jongh - if you do that, just keep the controller cool ... as Plonkster also pointed out, temp does affect it.

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15 hours ago, The Terrible Triplett said:

if you do that, just keep the controller cool ... as Plonkster also pointed out, temp does affect it.

I should start with relocating my panel to the roof this week somewhere? Will include racks for 2 x 150 Watts , once done I will split the systems and test this, If I can keep the bank above 80% I will leave it like that. 

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