Advice / thoughts please on AGM + LifeP04 combined for motorhome

[Bernie the bolt]

Hi newbie here with some solar experience ( installed own system on a narrowboat )
I have a 2001 Merc sprinter motorhome and I wish to upgrade its solar and home battery capacity to improve its ability for "wild" off grid camping .
Current system :-
Single 120w pv panel on the roof ( poorly wired by previous owner with excessive cable run and undersized cable )
Cheapie pwm Chinese ( purports to be mppt ) 40amp controller
Newish (3 year old) Leoch 120ah adventurer AGM House battery
Merlin smartgauge to monitor battery SOC
240v 15amp 3 stage battery charger ( also has a float setting)
Merlin VSR between starter and house battery. ( fitted with an disconnect battery switch as I suspect the slightly different chemistry causes a drain on the house battery when left connected over a long time when the van not in use )
Biggest drain overnight is the 12v fridge and blown air heating.
Typical overnight draw is 40 -50 ah resulting in battery soc @ 50-60% in the morning (typical winter values , summer is better )
Proposed improvements:-
Change the solar panel to a larger 250 - 350 watt with 4mm cabling via a more direct run to a true mppt controller.
Add a amp/hour meter with a shunt to better understand system loads / charge levels
Add some LiFePo4 connected in parallel to the AGM ( say 105 ah x 4 bare LiFePo4 cells connected in series with a smart bms + Bluetooth capability)

It's this last line I need advice on - what size bms should I order ? ( the sprinter's alternator has recently been upgraded to a 200amp model )
Currently the voltage under charge with engine running rises to about 14.75 v , I haven't checked the amps yet but I have a clamp multimeter rated to 200amp.
I also have concerns with the VSR , somebody has suggested the LiFePo4 voltage will "hold it open".
I was thinking of sourcing 4 x 3.6v LiFePo4 cells ( the blue ones ) from Aliexpress- about £107 for the 4 Currently with carriage from Germany, and a Daly BMS.
Any thoughts/ comments appreciated.
Bernard, Redruth Cornwall.

 

[A.Justice]

Cool van!

Assuming that the 105 ah cells are rated between 1-2c discharge rate (105 amps -210a), I would go with the 200a BMS. An overrated BMS will run cooler for the rated load (ex, a 75a load on a 100a BMS is 75% of it's max, and could definitely heat up quite a bit, that same 75a load on a 200a BMS is much less of a %). Getting a Bluetooth model will allow you to set your over discharge limits as well, so the 200a model could turn off if more than 150a (or whatever you set it to) are pulled.

Oversizing your BMS is never a bad idea, and I promise you won't regret buying a larger one.

I would also suggest isolating your new Li batteries from your old lead acid ones. I know there's a few videos floating around out there that say it's okay, but when dealing with an older lead acid bank, you don't know exactly how they're going to end up performing.

Will it (paralleling the two different chemistries) work? Probably. Is it best practice? Nope.

 

[Bernie the bolt]

That's great A. Will the bms also limit the charging amps ? I am concerned with a relatively small LiFePo4 bank the 200amp Bosch alternator could overheat / damage them.

 

[A.Justice]

That's great A. Will the bms also limit the charging amps ?

No. You can set it to disconnect if a certain charge limit is exceeded, but it's either on or off, and won't "throttle" the charge, other than preventing it from charging at all.

I am concerned with a relatively small LiFePo4 bank the 200amp Bosch alternator could overheat / damage them.

Most people use a DC to DC charger, or an alternator that's designed for lithium. I use a Renogy 40amp dc-dc unit that goes between the alternator and my LiFePO4 bank.

 

[mikefitz]

The manufacturer of your van suggests limiting the power taken by auxiliary circuits to around 50 amps. This needs a battery to battery charger. The upgrade to a 200 amp alternator may allow an increase but will still be limited. Most van alternators are not continuously rated and will only give rated current at above medium engine speeds, expect around 130 maximum amps at idle speed. Take into account the vans electrical systems and starter battery will consume around 50 amps with everything on.
Connecting the lithium direct vis a VSR will allow an unregulated current to flow, limited only by circuit resistance, and quite possibility be the maximum available from the alternator. This could overload the existing system. The additional disadvantage is that there will be no control of finishing the charge process, possibility resulting in battery over charge.
A maximum charge voltage of 14.2 are even lower at 13.8 to 14.0 is much less stressful for lithium and with 'economy' cells is less likely to cause cell overvolts and BMS shutdown. Float voltage should be 13.4 volts or lower.

Only a battery to battery charger will give correct charging, ideally a unit with user programmable charge values.
Chargers from, Victron, Sterling power, Votronic are available, European based companies with good customer support.

Dont charge via the PWM controller, use a quality 'real' solar controller, the Victron Smart 100/30.

For battery monitoring , Victron smart shunt or Victron BMV712.

With the battery consider upgrading to more capacity and removing the AGM from the system. Although its possible to run AGM and lithium in parallel its not very efficient, and I expect your AGM is past its best, Leoch batteries rarely last over 3 years without a significant loss of capacity.

With modest power requirements you dont need a 200 amp BMS. I would recommend a BMS from LLT,

4S or 3S 12V Li ion or Lifepo4 Battery smart BMS with Bluetooth function UART and RS485 communication with 60A to 120A constant current - LLT POWER ELECTRONIC

1) Application:this smart BMS can work for 3 to 4S Lithium ion or Lifepo4 Battery ,client can choose based on your battery chemistry features 2) Current : constant current of this version BMS can be made with 40A to 120A in accoridng to the different demand 3) Dimension: :Max 138*102*15mm for...

www.lithiumbatterypcb.com

This company take Paypal and if you opt for air freight there is a 7 day delivery time to the UK.

There are other variants of BMS available.

3S to 4S Li ion or Lifepo4 Battery BMS Archives - LLT POWER ELECTRONIC

this Type BMS was widely applied on 12V Li ion or Lifepo4 Battery Pack with 10A to 200A constant current ,which can be applied on various field such as Backup power supplier , solar energy Battery BMS ,DC to DC inveter Battery system ,street light BMS etc

www.lithiumbatterypcb.com

The type in the link is the Overkill BMS type so plenty of support. The Bluetooth app gives battery status and parameter setup. Recommended rather than a Daly product.

£107 or 4 off 105 Ah cells sounds too good to be true.

For your use I would suggest 200Ah of lithium battery. Consider getting a pre built battery , example,

Roamer - Premium LiFePO4 Leisure Batteries for off grid

For continuous power for non-stop adventures, say hello to Roamer – the next generation in SMART 12v and 24V LiFePO4 leisure batteries for campervans, boats and off grid life. Equal parts power, intelligence and reliability - managed with a smartphone app that puts the power in your hands. 10...

roamerbatteries.com

If you have more questions, I have some experience on van conversions in the UK and boat applications.

Question, that looks like an AC unit on the top of your van, do you need a high power inverter?

Since some/most of the electricals have been installed by someone else, take care to examine the system carefully. In my experienced undersized cables and inadequate or non existent fusing/breakers is common on the DC system.
Its even more common for the 230AC system to be incorrectly installed.

Mike

 

[Bernie the bolt]

Wow Mike , many thanks for your comprehensive reply. I will take the time to look through the links to the products you suggest.
Yes that is a 240v AC on the roof , we only use it when on campsites with 240v hook up , usually when in S.France or Spain.
The Leoch battery was installed by myself (along with the VSR and Merlin smartgauge at that time ) I have never let it get below 50% SOC and typically 65% is the normal. The poor solar system does at least keep it at 13.8v and 97-100% SOC when the van is not in use.
Therfore I assume the battery is still good. ( can you suggest a way to check the true remaining capacity ? )
As funds are limited now by our pensions, I think I will start a planned upgrade, starting with the solar system ( new larger panel , true mppt controller, proper wiring.) Leaving the battery upgrade to later.
My aim is to reduce the need to run the engine for charging when wild camping ( staying in the same spot for 2-3 days )
I notice some of the lighting is not LED ( some are still the 12" long 12v fluorescent fittings) so I need to upgrade those those ) .I have already changed all the mr11 halogen in the ceiling to LED.

 

[A.Justice]

I've ordered all but one of my BMS's from LLT. I have nothing but good things to say about them. I have a few of the 4S, 120 amp units that I've been very happy with.

I suggested the 200 amp BMS for a couple of reasons, NEEDING one of them is not one of them. ? Like @mikefitz said above, your loads are small enough that you probably won't need all 200 amps; BUT, in my personal experience, once you get a decent electrical system wired up, you'll probably start adding more things to it, and the extra "oomph" from the larger BMS won't be a bad thing for your future loads.

 

[mikefitz]

I assume the battery is still good

The battery monitor you have is not in common use, and to be honest I cannot see how it can measure the battery capacity unless it measures current flow. Perhaps with advanced algorithms it can compute state of charge, but SOC is only one factor in battery management. However from you comments, upgrading this area is one of your objectives.

I understand the limited funds and the increase of solar panel power be very cost effective. However the small reserves of battery power, 120 Ah with a practical user available 60 Ah, will only give just over one day, maybe two at a pinch, with poor solar conditions.
Note that the typical charge time for AGM batteries from 50% SOC to 100% will be a minimum of 5 hours regardless of the charger power.

voltage under charge with engine running rises to about 14.75 v

This seems high for an alternator, have you confirmed with more than one meter? Standard fit alternators of your year Sprinter , were usually lower falling to the high 13 volts after a time. 14.75 continually on the starter battery , and the service battery if its fully charged, may not be good news.

true mppt controller, proper wiring

If you have room on the roof, the roof bars and the AC unit limit space, the large domestic panels are good value,

295W Smaller Size Perlight Total Black Delta Mono Percium Solar Panel. Delivery from £33 - 54 cell smaller 1.5m size - great for vans and motorhomes, MCS Approved - The Installers choice - - 30 year parts & performance warranty

www.bimblesolar.com

4mm2 or better 6mm2 cable from panel to solar controller, 6mm2 or better 10mm2 controller to battery, install with a short cable run to battery.
You are unlikely to get more than 20 amps from a 300 watt panel flat on the roof, and in the event of higher yield the MPPT controller ( if opting for the 20 amp unit) will just limit the output .
Victron Smart solar 100/20 at £140 or 100/30 at £210
Budget alternatives, Epever XTRA 2210N 20 amp, £70 EPever XTRA 3215 30 amp £115

My aim is to reduce the need to run the engine for charging when wild camping

Not good for the engine and with the 120A agm the charger rate will be slow.

Your van model had the option of the 5 cylinder engine, do you have this?

can you suggest a way to check the true remaining capacity ?

A very approximate test but will give you some idea.
Your monitor should , says the literature, display true state of charge.
charge the battery, load with a car headlight bulb, 55 watts, time until the SOC is 50 %, stop the test. After a rest of 1 hour the battery should read slightly over 12 volts, recharge the battery.
The test does not have to be continuous.
Example the test runs for 10 hours, watts used 55 x 10 = 550 watts. 550 / 12 = 46 amp hours, thus capacity is 92 Ah.

You could get a slightly more accurate result with one of these,
hall effect power monitor

Mike

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[Bernie the bolt]

Your van model had the option of the 5 cylinder engine, do you have this?

I wish ,no its the 109hp 4 cylinder 311cdi , a good workhorse though with only 170k on the clock , I am told they are good for 500k if looked after.