Simple MPPT Setup Questions


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gpn

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Jan 21, 2018
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250
I have some charge controller questions that I can't seem to find via search.

I just finished the beta testing phase for a small portable power box. Beta means it works but just need to finish the little details...

I have a victron 100/15 mppt in the box for charging the 4s10p lifepo4 pack. What should I see absorption and float values to? Right now I have absorption at 14.2v and float to 14.21v. I don't think that I can control the bulk setting but there is a "re-bulk voltage offset" which is set to .1v right now. Absorption time is set to fixed 2 hours at the moment as the suggestion of victron.

Otherwise things are looking good. I'm putting 100w of solar into this little battery. Should be all charged up soon!
 

jonyjoe505

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Feb 28, 2018
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229
Absorb and bulk are the same thing, its the highest voltage the battery will get before switching to float.It will be trial and error getting the right bulk setting. If you have voltage drop from controller to the battery 14.2 volt will be too low. I have alot of voltage drop on my system 4s40p 220ah lifepo4, Im using a differentmppt controller (ecoworthy 20 amp) 240 watt panel.

I have to set the bulksetting to 15.5 volts and float to 14.3 volts(highest it will go). Thats the only way to get maximum amps (around 12 amps) to the battery. If I set the bulk to 14.6 volts, it will charge at less then 6 amps, it will never fully charge.

Your going to have measure the volts on the controller and bounce that with the voltage you get from the battery terminals. If you get 14.0volts on controller and only 13.5 on terminals, that means you have .5 volt voltage drop, so you will compensate by raising bulk setting .5 volts.

If you plan to use a regular bms on the battery, don't connect any devices to battery while charging. If the bms triggers to stop charging you will get voltage surges and destroy any 12 volt devices that are connected. Another thing you can do is connect all devices through a 12 volt voltage stabilizer (cost anywhere from 20/30 dollars) but these only handle less then 10 amps but theywill protect your devices. Voltage surges are very real, even if you plan on never fully charging the battery, if it goes out of balance or bms overtemps it will stop charging and the controller has no where to dump the power except into voltage surges.

The only bms I recommend for charging with solar is the chargery bms8t, this will disconnect the solar panel (with a mechanical relay) , this prevents voltagesurges but cost is about 100 dollars, well worth if you want a high quality bms. This will protect your expensive lifepo4 batteries and devices, also has audio alarms, temps sensors and 1.2 amp balancing.
 

OffGridInTheCity

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Dec 15, 2018
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2,032
winny said:
jonyjoe505 said:
Absorb and bulk are the same thing,

No. Bulk is CC mode. Absorption is CV.

Bulk is all the current that is available till the target voltage is reached. Remember that a charger sets v higher than the battery so current will flow TO the battery.Absorption is Constant Voltage (a bit higher than battery)but reduces current gradually till target v and battery are close. Float is similarly Constant Voltage to maintain the v after absorb does its job.

Typically, for Lithium-Ion, since you don't care about Lead Acid 'memory' and other Lead Acid sensativities- you simply set Absorb/Float as the 'max charge voltage' you want for your battery. In my case (14s 18650) I shoot for 56.0 (4.0v/cell). The Controller's idea of voltage vs reality is usually off a bit - so that you may need to set 56.2 or 55.9 to actually get 56.0v (4.0v/cell). This is where a good/base voltmeter comes in - so you can determine the actual voltage you believe to be the most correct.

For Midnite Classics I set Absorb to 56.1 and Float to 56.0 because itwon't allow me to set Float >= Absorb. The 0.1v differencedoesn't matter that much as long as my battery tops out at 4.00v +/- .05 The Absorb vs Float is not of concern... as the combination simply causes the battery to top off at 56.0v - which is what I want and does not harm the Lithium-ion.
 

gpn

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Jan 21, 2018
Messages
250
Well now I'm more confused. So since the victron doesn't have a bulk setting I set the Absorption to 14.2. That is 3.55/cell. So it will bulk (CC) charge to 14.2 (or maybe a little less) and then switch to CV mode, which in LA is Absorption. The big thing is keeping the float turned off. Is that correct? I can turn off float on this controller.
 

OffGridInTheCity

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Dec 15, 2018
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2,032
gpn said:
Well now I'm more confused. So since the victron doesn't have a bulk setting I set the Absorption to 14.2. That is 3.55/cell. So it will bulk (CC) charge to 14.2 (or maybe a little less) and then switch to CV mode, which in LA is Absorption. The big thing is keeping the float turned off. Is that correct? I can turn off float on this controller.

Bulk is the default behavior till voltage reaches Absorb (14.2). Then Absorb takes over. That's why there's no 'bulk setting'.

>The big thing is keeping the float turned off. Is that correct? I can turn off float on this controller.
No - You setFloat to same voltage as Absorb (as close as it allow you). Then, when Absorb finishes topping off the battery - Float will take over and kick in/out as needed (provide chargingcurrent)to keep the battery stable at the given voltage until .... you draw power out of battery and bring it so low that bulk / absorb / float cyclekick in again.

Maybe this will help
1) Bulk - maxcharge (max current going into battery). Note:Some controllers let you control the max current in case your battery is too small or wires to small or other reasons.
2) Absorb - max charge -> gentle charge to 'top off' at given v(current reduces gradually till you top off the battery).
3) Float - maintain the battery v. (you could view this as a very gentle form of Absorb)
 

gpn

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Joined
Jan 21, 2018
Messages
250
Ok. That is helpful. I can control the max current. I have it set to the controller max of 15 amps. The pack is made from k2 26650 lifepo4 cells in 4s10p setup. So even if I get to max charge current (my little portable panels only do ~120w) I'm still only putting in 1.5amps per cell which is less than .5C.

I'll set float to the same as absorb. Otherwise should be good to go. Thanks again!
 

Korishan

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Jan 7, 2017
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Ohh!! LiFePO4 cells!! That makes a big difference on how to handle these cells. You don't have to be so gentle with them like you have to with Lithium Cobalt or the other "4.2V" cells. LiFePO4's are very forgiving on the charging. You could probably do even 2A per cell and be fine.
 

gpn

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Joined
Jan 21, 2018
Messages
250
I had to double check... And yes I did put the chemistry in the first post. Phew.

In terms of charge rate the limit is definitely the equipment and not my cells! I'll have to add some pictures once I tidy up the wiring. I was just doing some testing with it the other day to see if it worked before I made it prettier.
 
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