12v Trigger to DC-DC Charger Wiring Mistake Leads to Dead Starting Batts

[porschedpm]

I made a stupid mistake last week that resulted in nearly completely draining my starting batteries in less than 2.5 hours. Both starting batteries were fully charged and about 9 months old. Here's my story:

Three years ago, on my 2006 RB50 SMB, I abandoned the Battery Separator altogether, ending up with a completely separate system for the 680ah Lithium coach battery bank and a separate system for the 2 lead acid starting batteries. A Renogy 40amp DC-DC Charger is the only link between the two battery systems. When I installed the DC-DC Charger 3 years ago I wired a 12v lead from the right rear running light to the DC-DC Charger so that the DC-DC Charger would activate whenever I turned on the running lights. This system worked good for years since during the day, when I do most my driving, I had control over when to use the Alternator via the DC-DC Charger to charge the lithium house batteries. But when driving at night, whenever the headlights were on, the Alternator was always feeding the lithium coach batteries which I felt was less than desirable.

So to give me control, night and day, I decided to trigger the DC-DC charger from a 12v lead running through a rocker switch instead of from the right rear taillight. My first big mistake is that I tapped the 12v from a circuit that's always on rather than a circuit that provides 12v only on the run side. My second big mistake was that I forgot to turn this switch off after stopping last Saturday to see my wife's aunt and uncle in Hollister.

After visiting for 2-1/2 hours, when we left to leave I first found the auto door locks were acting wonky. Then found the starter didn't even start to crank. The starter battery voltage had dropped to less than 4v. In addition to the me negligently leaving the DC-DC Charger switched in the on position, a contributing factor was that when I parked, the coach batteries were at about 65% SOC so they were very thirsty. Without the alternator running, the large lithium battery bank sucked the life out of the starting batteries. After depleting the starting batteries I noticed the SOC of the lithium batteries jumped up to 84%. Fortunately I was able to run jumper cables from the coach batteries to the starter batteries and avert disaster but it was a couple of lessons well learned. #1-Never trigger a DC-DC Charger from an always on circuit, even if a switch is used...too easy to forget, especially at my age. #2-Never underestimate how much juice a Lithium batteries can absorb especially when SOC is down.

[marret]

Ouch, glad you survived to tell the tale, hope the start batteries did as well. Thanks for sharing.

I thought one could program the DC-DC charger to only charge if there is a certain voltage or amps available. May depend on the charger.

[Scalf77]

First thing I would check is the manual of DC to DC charger. Many work as marret says, by voltage level, much like the ACR or Separator . Some will have the ignition input for a smart alternator input, this will actually lower the input voltage it will operate at. It also could just be trigger enabled like you think, but this is not the norm.

Actually an easy place to find a "hot in run" point is at the steering column ignition switch connector.



This is where SMB usually get's there connection.

And in reality this isn't really a Lithium Battery issue. if you had a AGM for your house , the DC to DC charger would still have put out the bulk amps until the charger went into absorption stage. In either case the result would be the same.

-greg

[posplayr]

Originally Posted by porschedpm

A Renogy 40amp DC-DC Charger is the only link between the two battery systems.

The specification for the Renogy DC-DC charger is: Input Voltage Range: 8V-16V

So it is more than disappointing (i.e alarming) that the charger when activated will drive the source battery to oblivion by fully discharging well below 12V. I never like the fact that this DC-DC charger would even function down at 8V but even worse this doesn't even seem to be a charging limit!

As a rule of thumb, you don't want your start battery to be discharged which means you are below 12.8v (at the start battery terminals). However, in my own calculations of current drop and wire sizing for a 40A Renogy located in the rear of an E-350 with a Lithium battery, the numbers work out to about 13.5V at the start battery terminals else the current demand and voltage drops become unwieldy.

In the table below you can get an idea of how current demand goes up at lower Alternator outputs translating to lower start battery terminal voltages. The table was generated using 3% nominal voltage drops in all wiring.

The only time you would want to be DC-DC charging at idle is in the 20 amp mode. At 40A charging and 13.5V at the start, battery losses are exceeding 3%

Summary Analysis (sorry for the poor formatting the website pulls out blank spaces)
________________________ 40A Charging_____20A Charging
______________________Hi RPM___Lo RPM___Hi RPM___ Lo RPM
DC-DC Demand_____A___40________40______20_______20
Start Battery_______V___14.6______13.5_____13.5_____12.8
DC-DC_IN_________V___14.17_____13.04____13.27____12.5 6
Alternator Demand__A___51.5______56_______27.5_____29
Voltage Drop_______%___2.93_____3.44______1.7______1.88

[porschedpm]

Thanks everyone for your comments and support. Will definitely be tapping into the A2 position on the steering column ign switch connector. I figure I probably took at least year off the starting batteries' lifespan. I'm hoping not much more since the batteries didn't have much time to sulfate and I was able to recharge right away.