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Neil’s Lifepo4 system design discussion

PostPosted:Fri Jul 19, 2019 8:06 am
by NeilV
Hi all,

So I have a somewhat complex customised primarily 24v AGM house system already, and am now switching my house setup to run from Lifepo4 and looking to use most of the things I have already.

As I live fulltime with my wife and two kids, I need to run my Samsung 254L fridge 24/7, and a 5KW diesel heater I needed a fairly big setupwanted South Island resilience

I already have DVSR alternator charging (@65A) and a 40A Tracer 4010an solar controller with 780W of panels (in parallel). I had 2x100ah AGM (800CCA) for cranking (3year old house batteries repurposed) and 2x130AH AGM for house batteries.

This made a good maintenance free setup as the AGM shared a common charge profile and ‘just worked’... until I killed the starter AGMs by cranking them mercilessly when trying to get my engine going consistently after a breakdown (in the end it was the diesel injector pump :roll: )


Re: Neil’s Lifepo4 system design discussion

PostPosted:Fri Jul 19, 2019 8:43 am
by NeilV
So what I have at the moment is my whole bus running on the 130AH AGM’s, the fridge switched off overnight (Diesel heater takes preference in 6C temps!) and have to run the engine/alternator every overcast or rainy day to ensure I don’t spiral down below my self imposed 80% SoH (or a non functional starting bus!)

I have now bought the 200AH LiFePo4 Cell’s that Chris had spare (Nut17) and will be using them in 8s (8 series cells) as a (nominally) 24v 100Ah house battery.

After much research Forum/viewtopic.php?f=55&t=19099&sid=3f ... 087c7c1389 I have now bought a few extra devices too... namely

a Victron Battery Protect ... prod_image

A ‘Daly’ 60A Battery Management system (60Ax26v=1600W)

2x HA02 battery equalisers [4s and 10A]
After advise from Nut17 and his son Gadget suggested the Daly BMS had limited cell management due to its much smaller mosfets)

Re: Neil’s Lifepo4 system design discussion

PostPosted:Fri Jul 19, 2019 8:54 am
by NeilV
What I’m hoping to do is use the AGM as ‘starters’ and leave the DVSR in place for alternator charging (28.2v 65A=1800W) and set the solar controller to charge at the LiFePo4 rate.

The DVSR will essentially parallel the batteries until the LiPo are 30% and disconnect the AGM (95%SoH).
(Connect 26v (2min delay) LiPo 30%, AGM 95%
Disconnect 25.6v (5 min delay) AGM 100%, LiPo 20%
Ove Voltage Protect 32v (no delay)

Re: Neil’s Lifepo4 system design discussion

PostPosted:Fri Jul 19, 2019 2:19 pm
by NeilV
So I’m looking at having 1060W solar (1 more 280W on the way) for (in ideal weather at peak sun) up to 40A charging (@24v) in the Daly BMS’ second ‘charging’ input, and up to 65A ([email protected]) from the alternator (big bus alternator is capable of full capacity for the hour or so it would take from flat)

This means that rain or shine I’ll have the batteries filled in an hour of driving or 4 sunshine hours, and have an effective useable80Ah (2kwh)for the ‘house’ and the AGMs top 200WH/5% (before DVSR breaks the link) to boot!

Yes the AGMs will always be ‘topped up’ by the LiFePo until the Lifepo is at 30% and the DVSR disengages, but Lead acid love to be ‘full’ so unless the lithium’s 28A ‘float’ profile (mine will be 26.6v or 90%for Lifepo4 )is too high for AGM ‘float’... (and the default for sealed is 27.6) what could go wrong? :lol:

Re: Neil’s Lifepo4 system design discussion

PostPosted:Mon Jul 22, 2019 7:56 pm
by NeilV
From a 2015 post on Gel/agm hybrid charging, you say you had a variable alternator setup?

I’m VERY keen to figure a way to limit my 65A alternator to roughly 50A and retain the option of alternator charging my new to me Lithium’s :D
Neddy wrote:
Thu Sep 24, 2015 6:15 pm
I once fitted a home-brew manually variable alternator controller to my boat. I could crank the charge current up to 80 amps, no trouble.
Since lithium’s (practically zero) internal resistance means it will ‘absorb’ whatever it is presented, I am currently worried that roughly hour of _full draw_ on my 65A alternator will soon kill it, and that should I put in the 220A alternator that has been offered “free to me” by my brother-in-law it would charge them at 2C and soon kill the batteries instead!

Could you explain how your ‘home-brew’ managed a variable input as this seems to be exactly what I’m trying to achieve! :TU

Re: Neil’s Lifepo4 system design discussion

PostPosted:Tue Jul 23, 2019 10:39 am
by Neddy
Like most variable alternator regulators, mine used a power transistor to control the alternator Field current, but even a simple rheostat can provide full manual control over alternator output. I don't believe that would be a good way for you to go, though.

For $250 you could buy a locally manufactured ZM4 variable alternator regulator, but given your situation that is not an ideal solution either.

The best option would probably be fitting a DC-DC charger such as those made by Redarc or Sterling.

Another (slightly cheaper) alternative would be to go for a BMS system that communicated directly with the alternator. For example, Victron have a $476 version that could restrict alternator output to 50 amps or whatever limit you set, thus taking good care of your alternator and your Li batteries.


Re: Neil’s Lifepo4 system design discussion

PostPosted:Tue Jul 23, 2019 11:17 am
by Nut17
I would hook it up and test it before trying to limit the current from your alternator. As (from the way I understand it) the alternator will only be supplying current to the LiFePO4 bank when connected in parallel with your start batteries it would be prudent to actually measure the current actually being delivered to the lithium cells. I will be surprised if it gets anywhere near 50 amps.

Re: Neil’s Lifepo4 system design discussion

PostPosted:Tue Jul 23, 2019 3:55 pm
by scubadoo
No need to spend big money Neil.

My 100A rated alternator originally pumped 90A into my battery pack until full. I felt sorry for the alternator.
A very simple method to reduce current is to insert a tiny additional resistance in series with alternator output to the battery.
I initially inserted less than a metre of 8G cable in the circuit and from memory the current dropped to nearer 65A. The added cable barely ever reached luke warm. There will be no fire. :TU Ohms law tells us so.
Unfortunately the terminal voltage , 14.5V in my case, when the battery pack is fully charged does not alter so I now reduce that a little with a diode block but even the small lengths of 8G wiring added to complete the circuit lower the output by about 10A.
Once you have installed the new battery pack and if you consider the current may be a strain on the alternator experiment with a little series added cable length until you are happy with the charge rate.