How to maintain the Start batteries while inactive?

Peterbylt

New Member
Joined
Oct 20, 2020
Messages
15
This is in an MCI Bus Conversion.

The issue I am having is with the Bus lead acid start batteries, these are two, 8D, 12 volt batteries wired in series for 24 volt.

In between uses the bus sits in a storage area with no access to power, Inactive time could be up to a couple of months, the Bus batteries slowly loose charge.

I am also building a solar system using the Electrodacus SBMS0, The house batteries will be, 24 volt, 8 302 Catl Lifepo4 cells, 1500 watts solar in 6, 250 watt panels, these will be set in 3 groups of 2 parallel panels. Each Group of Two will output an Approx. Max of 30 volts and 17 amps.

The SBMS0 does not accommodate Lead Acid Batteries.

I am thinking a possible solution is get a separate charge controller maintainer and wire one set of the Solar panels that can be switched from the SBMS0 to the separate charge controller to maintain the 8Ds while the Bus is inactive.

The current plans call for the Lifepo4 batteries and the house system to be disconnected while the Bus is Inactive and unattended, my understanding is the Lithium batteries can maintain storage much better than the Lead Acid batteries.

Does anyone have a better suggestion?

Do you have any recommendations for a long term 24 volt solar powered Lead Acid Battery maintainer?

This is one possibility.

https://hqsolarpower.com/20a-mppt-solar-charge-controller/

Peter
 

Sirtate

New Member
Joined
May 27, 2021
Messages
21
A small DC-DC (like a 20amp one) charger can keep the Starting Batts topped off using the House Batts for power...
As mentioned earlier, it doesn't take much to float a FLA batt
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
6,473
Location
Somewhere South of Denver
Keep the LiFePO4 system active, but lower the charge profile to 14.0v or whatever you're comfortable with. Add a 24v->12v DC-DC charger from the LiFePO4 system to the lead acid system.

My LiFePO4 battery bank stays in my RV trailer year round. I lower the charge during storage and bump it up a bit when I'm actively using it.
 

Horsefly

Photon Sorcerer
Joined
Dec 12, 2020
Messages
1,015
Location
Denver, mostly
Add a 24v->12v DC-DC charger from the LiFePO4 system to the lead acid system.
Good suggestion up to this point. OP said the starter battery is also 24V, so 24V->24V DC-DC charger instead. Otherwise, I agree with your suggestion.
 

Peterbylt

New Member
Joined
Oct 20, 2020
Messages
15
All good suggestions, I was really trying to keep the house system from being part of the equation, I do like the idea of lowering the Charge Profile to keep it from cycling too much, I could also wire in a few well-placed disconnects to remove the house systems and any parasitic drains.

I could also with some creative wiring use the 24 to 24 charger to go the other way and charge the house batteries from the start batteries in some conditions.

I have also been toying with the Idea for a direct connect Jump Start switch for emergency circumstances.

On a side note, the field of 24/24 chargers is very limited.

Peter
 

pollenface

Solar Addict
Joined
Nov 14, 2020
Messages
494
Location
Perth, Australia
I maintain a collection of lead acid starter batteries (all in parallel) with a 240v maintenance charger which runs from the inverter via a daylight switch (10w solar panel and a couple of relays to switch the power on/off from the inverter during good sunlight hours - but that's another story)
 

rickst29

Solar Addict
Joined
Mar 27, 2021
Messages
341
Location
Reno, NV
Your 'direct connect Jump Start' should probably be running through a 'pre-charge' resistor circuit first, in order to keep the sudden load on the LFP house batteries from exceeding the BMS current limits (when the lower-voltage 'starter battery' pair is suddenly connected to the LFP pair). Such devices are widely used in conversion vans.

The rest of this is VERY complicated, but maybe worthwhile: For charging from one set to the other, you can use a DC->DC Boost Converter to make the source battery appear as a Solar Panel, and then let an MPPT controller convert the high voltage down to service the "target" battery charge needs.

In my scheme, the "source" power bus would be run into a 24->36 Volt Boost Regulator. (In all the installation I've done, that boost is only 12->36 or 12->24, with 12v battery banks.) If there aren't any high-power converters available for that conversion, you could also boost to 48V - I know that 24->48V boosters are readily available.

Then, with the target battery connected to an MPPT Solar Controller as the "battery", you connect the Booster output as the "PV array". Program the MPPT controller output parameters to match the target battery string. It will "see" a 48V solar panel. Within the MPPT Solar Controller, you also program a current limit which will limit "maximum demand" from the Boost Converter, to assure that the Boost Converter isn't burned out by over-utilization from the MPPT input side.

In my own Travel Trailer example, I'm going in the "Tow Vehicle" to Travel Trailer "LFP" direction. My 12->36V Boost Converter is rated for a maximum of 15A output (540 watts), and I don't want to run it harder than 85% of capacity (459 watts). There are also some wiring losses to reach my Trailer-Mounted MMPT controller, so I don't want my MPPT trying to pull more than about 430 watts. With my LFP batteries accepting up to 14.4 Volts in "Boost mode", and the MPPT only 95% efficient in converting from ~35.5 volts to 14.4 Volts, my maximum amperage calculation is 430W / 14.14V = 29.86A, and then 29.86A * .95 efficiency = 28.3 allowed (as output to the LFP battery string). I therefore set my MPPT for a maximum of 28A output current.
- - -
You would run this backwards, with "Boost" activated at the LFP batteries (~ 24V to 48.0V regulated output) and the MPPT programmed for FLA batteries. Because the MPPT wouldn't like the hostile under-hood environment of the bus, you might not be able to attach a BTS -- but with fairly low current, and with a conservative maximum timer for "Boost" mode, you'll be able to prevent overheating the FLA batteries anyway. If the bus is typically under good sunlight, even in storage, you can leave this switched "on" all the time.

In my configuration (running the other direction), I use a dashboard switch to engage the Boost Converter only when I'm actually driving (with adequate RPMs for the alternator to create those "extra" watts and amps). With enormous complications of added relays and control switches, you could make yours bi-directional -- but only with the MPPT output configured as a compromise, safely meeting the basic requirements for both FLA and LFP at the same time. (Maybe "Boost" reduced to only 14.3 or even 14.2 for up to 30 minutes, NEVER "equalize", and switch from "CV" of about 14.2 to "float" of 13.5V when that voltage is reached.)
 
Top