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Re: Neil’s Lifepo4 system design discussion

PostPosted:Mon Jul 29, 2019 8:56 pm
by NeilV
Finally, the tricky AGM starters and alternator charging issue has been backbenched with a 60A fuse/switch that can parallel them if ever needed.

Unfortunately the Victron alternator controller is 12v only, so no good for my setup, and a B2B is beyond my current means, so I’m going solar only and will see how if I need alternator charging in the end anyway.

Also of note, today at 2pm in full sunshine I saw (first time ever) a 0A input with a 35.6V solar reading, as my ‘boost’ setting took the lithium’s to 27.2, and the float was 26.7... and they were just floating... while the fridge was chugging away and the solar was on float BELOW the LifePo4 flat voltage at 100% (27.2v) :lol: :lol:

Re: Neil’s Lifepo4 system design discussion

PostPosted:Tue Jul 30, 2019 7:56 am
by Nut17
Good news Neil. :TU

Re: Neil’s Lifepo4 system design discussion

PostPosted:Tue Jul 30, 2019 8:38 am
by Neddy
NeilV wrote:
Mon Jul 29, 2019 8:56 pm
My tricky AGM starters and alternator charging issue has been back-benched with a 60A fuse/switch that can parallel them if ever needed.
One of the more important occasions when such a switch would needed would be if your AGM starter batteries ever failed, were inadvertently flattened, or had insufficient charge to crank your engine. In such circumstances, a 60A fuse/switch link paralleling Starter and House batteries would be woefully inadequate and blow just when you really needed it.
Keep in mind that such a link could be called upon to carry full engine cranking Amps and needs to be sized with this in mind.

Neville.

Re: Neil’s Lifepo4 system design discussion

PostPosted:Tue Jul 30, 2019 10:40 am
by NeilV
Neddy wrote:
Tue Jul 30, 2019 8:38 am
One of the more important occasions when such a switch would needed would be if your AGM starter batteries ever failed, were inadvertently flattened, or had insufficient charge to crank your engine. In such circumstances, a 60A fuse/switch link paralleling Starter and House batteries would be woefully inadequate and blow just when you really needed it.
Keep in mind that such a link could be called upon to carry full engine cranking Amps and needs to be sized with this in mind.

Neville.
Thanks Neville,

I previously had a 250A switch/fuse and the 200A ‘DVSR’ and both worked well when I needed to use the house to ‘jump’ the starting before (unknowingly left the ignition on when parked up)
What I did was to parallel the two sets first for 1/2 an hour (had a coffee) to let them both equalise before I went to start the engine again, as I felt that would be kinder to the starter batteries too, and this seemed to work well?

I think as the Lithium’s have such a higher voltage state than the AGM I’m now using to start, this would work even better now, as 26.2v on the lithium house batteries is their 40-70% SOC and that’s a great ‘charge’ voltage for the AGM?

The main purpose for the 50A there is to restrict the alternator from being able to charge the lithium’s at it’s full capacity for long enough to damage either the alternator or the lithium.
As 50A is my idle alternator output, if it was needed to charge the ‘house’ I can idle to do so, but the trip will drop the link if I accidentally leave it connected when I drive off and the alternator goes up to its full 65A output?

I repurposed the 250A as a main cutoff switch for the whole house setup, as both the BMS (60A limit on house negative) and the Victron battery Protect (65A limit on house positive) will protect the house from a short.

I guess I can shift the switches around if needed, and I did make a separate 50cm x 50mm cable to be able to ‘jump’ the two also if ever I need to :TU

Re: Neil’s Lifepo4 system design discussion

PostPosted:Wed Jul 31, 2019 8:43 am
by Nut17
Neil, have you actually tested the system - ie run the engine with both battery banks in parallel and measured the current being delivered? The internal resistance in the LA starting batteries may well limit the current. I am no expert, but it may well be worth trying. When my DC - DC charger failed back in mid 2012 I did connect my direct feed from the Jeep battery to my 200 AH LiFePO4 battery, effectively paralleling my start and house battery. The Overland was fitted with a 235 Amp alternator but was only delivering around 48 amps to the house battery. The relatively long cable run would have had some limitation but there was 4 gauge cable used for the installation.

Re: Neil’s Lifepo4 system design discussion

PostPosted:Fri Aug 02, 2019 8:18 pm
by NeilV
Spot on Chris!

When running my alternator I can now flick the 50A tripswitch and see 20A flowing into my house system on idle, and 30A on revving/driving, about 26ish Amps entering my Lithium’s (I have separate shunts on the starter’s and house) this is explainable as the fridge was on at the time too, so drawing off 4A for its load.

The voltage also dropped from 28Von the 1.2A trickling into the AGM starters to 27.5v when they were in parallel.

As the lithium charge rose its voltage also rose (slowly) and the alternator Amps tapered off to around 10A at 27.85v (26.8v lithium voltage when disconnected) after about an hour drive.

This is now a perfect solution, as long as I remember to disconnect when it gets to the 10A dribble, or when I stop... as the Lithium’s at 26.8 then trickle charge the AGMs at _their_ perfect ‘float’ voltage :lol:

(I have 50mm cable in the parallel circuit so almost zero loss there too)

Re: Neil’s Lifepo4 system design discussion

PostPosted:Fri Aug 02, 2019 8:23 pm
by Nut17
It is often too easy to complicate things unnecessarily. A good result. :TU
You could simply fit an ignition switched relay between the battery banks so they are only in parallel when the engine is running.

Re: Neil’s Lifepo4 system design discussion

PostPosted:Fri Aug 02, 2019 8:58 pm
by NeilV
On a different note, I now have my fridge 300W inverter on the ‘load’ terminal of my 40A Tracer solar controller, as I noticed that after doing a ‘boost’ (set it at 20min and 28.2V) charge at about 13A in good sun last weekend, it then dropped to the set float of 26.8V (roughly 90% SoC point for LifePo4) and the solar went to 0A and 38v (open circuit) while the lithium started draining out the 5-6A the house was then drawing down :o

Since my fridge is the main draw, I wanted the batteries to stay ‘ful’ until the solar went off at night, so by putting the limited 300W inverter that supplies it onto the solar ‘load’ circuit (300W/24=12.5A max) it now draws straight from the sun while sun is able to supply...

As it still registers on the Shunt when drawing back from the batteries at night and allows a timer to turn it off overnight and on again the next morning- in winter (when my solar performs worst and the fridge is so easily iced :lol: ) this seems like an ideal setup!

It also shows that the 300W inverter has a 0.3A standby draw.

Really funny to hear my budding solar enthusiasts (sons are 8&10 now) bewildered at how there was NEGATIVE 2.6A early today when the panels were showing 2.3A produced (as the controller was drawing from the batteries to supply the fridge load) :-T

Bring on summer!