question

safe-harbor-haulers avatar image

Need advise on a system

I'm replacing the inverter in my tractor with a Multiplus-II and am looking for advise for the best way to go with my current battery banks. The truck came from the factory with 2 battery banks, The "truck" side consists of (4) Lead Acid group 31 batteries. The "house" side has (4) AGM group 31's. I'm sure you see the problem already.... I'm changing out the "truck" bank to AGM's.

They are connected through a solenoid, or combiner, controlled by a battery management system (BMS) that consists of shunts on either battery bank. When I am driving down the road the BMS closes the solenoid and charges both banks through a 300 amp brushless alternator. When I'm parked the system runs off of both banks until the truck side falls to 12.6 volts. At that time the BMS opens the solenoid, disconnecting the truck bank, and continues to run off the house bank until they are depleted. Once depleted the BMS then auto starts the truck, bumps the idle and runs for approximately 1 hour to recharge the batteries as much as possible and the process starts over.

Besides the obvious mixing of battery chemistry that I am resolving by switching out the truck batteries with matching AGM's does anyone see any other issues I may have? The reason for all this is I have frequent issues with the lead acid batteries and my original inverter (Xantrex Freedom HFS 2000) just fried. I'm concerned with the house batteries being depleted far more than the truck batteries, then through either the alternator or the shore power of the inverter, overcharging the truck batteries.

Thinking of:

(4) Group 31 Interstate AGM-7's

Rewire both battery banks to Inverter with 4/0

MultiPlus-II 12/3000/120-50

CerboGX

GX Touch 50


AGM Battery
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9 Answers
snoobler avatar image
snoobler answered ·

You aren't mixing chemistry. You are mixing construction methods. They are both lead-acid, and it is a minor concern at best. Charging schema for FLA and AGM easily overlap. If you are charging to a compatible absorption and float voltage, I see no concerns. I would not replace them unless the FLA batteries are worn out.


Properly maintained FLA batteries will almost always outlast AGM batteries.


Would recommend the addition of a BMV 702 for current and voltage sensing and accurate State of Charge (SoC) reporting.


Something that will impact battery substantially is proper temperature compensation when charging. When batteries are cold, they should be charged to a higher voltage, and when warm, they should be charged to a lower voltage. At 77°F a battery that should be charged to 14.4V should be charged to 15.15% @ 32°F.


Secondly, preventing the house batteries from discharging below 50% will also improve their lives. The BMV 702 actually counts current in and out of the battery and computes it's actual state of charge as voltage is very unreliable for that purpose. Hopefully, the BMS can be adjusted to optimize battery life.


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safe-harbor-haulers avatar image
safe-harbor-haulers answered ·

I always loose one or two lead acid batteries from the truck bank each year. I'm assuming from overcharging. The house AGM's are fine and have been in for the past 4 years. They are being charged from the truck alternator so I'm not able to control the charge voltage. Am I able to use (2) BMV 702's, one on each bank, and connect both of them to the Cerbo? I don't know how the solenoid connecting and disconnecting the banks will effect the SOC data?

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snoobler avatar image
snoobler answered ·

IMHO, no BMV on the starter batts. You can if you want.


Disconnecting them at 12.6V should ensure they're still at a very high SoC when it transitions exclusively to house batteries.


The chassis bank won't affect SoC because the BMV on the primary (-) at the house battery is measuring net current to/from the house bank only. It will always report house bank SoC accurately. The chassis bank is just "bonus" capacity that probably won't contribute much overall.

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safe-harbor-haulers avatar image
safe-harbor-haulers answered ·

Thank you for your input.

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kevgermany avatar image
kevgermany answered ·

If you're losing batteries out of the starter bank, consider overcharging as a cause. Alternators charge at fixed voltage, unless you have a modern smart alternator. This means the batteries are at max charging voltage while the engine is running.

Probably worth looking at what the alternator is doing, consider either modifying it for variable output or replacing it with a smart version. I can't remember the name of the kit that often gets recommended here, though. Sorry.

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safe-harbor-haulers avatar image
safe-harbor-haulers answered ·

The alternator is a 300 amp brushless, internally regulated, non adjustable unit. I am looking into either modifying it or replacing it with an adjustable externally regulated unit. I would then wire it to the house battery bank and use a DC to DC charger to charge the truck bank. Anyone with any input on this?

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Nickus de Vos avatar image
Nickus de Vos answered ·

What size Multi II are you looking at installing, what are your power requirements?


Have you got anything else like heaters, fridges etc. running straight off the 12V house bank, or does all your stuff run off AC?

Have you considered replacing the house bank with lithium?


I'd look at completely separating the truck bank and house bank, in other words only use the truck bank for starting and running things while driving, while the motor is running, don't use the truck bank for heavy currents at all while parked.


Let the alternator charge the truck bank, then install multiple Orion DC-DC's to charge the house bank while driving.

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Nickus de Vos avatar image
Nickus de Vos answered ·

If you totally separate the truck and house banks I'll also look at maybe going with a 24V, even 48V house bank rather than 12V.

I see you plan to install a 3000VA Multi, the rule of thumb is to try keep your amp draw on the DC side under 100A, with a 12/3000 Multi, you'll be drawing 250A, with a 24V Multi half of that.

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safe-harbor-haulers avatar image
safe-harbor-haulers answered ·

Thank you for your input. I appreciate it.

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