DC2DC charging and dead lead, what happens? Best FLA for it?

[CaptainRivet]

Hi all,
What will actually happen in DC2DC charging when the lead starter is dead?
At first nothing i guess, it still works because the DC2DC is taking the power from the alternator and not from the battery. So no problem....BUT
- if its an AGM that ran dry or got a cell short due to sulfaltation it will start gasing and get a thermal runaway= your boat will burn =no alternator charge..experienced that already, never AGMs on a boat anymore
- if its an FLA that has a cell short it will gas/cook but if you have cheap refillable ones you can fix that by refilling. maintenance free not and you risk it exploding if the venting holes are blocked or burning. So cheap refillable FLA superior for DC2DC.
This are just the 2 events that a dead lead will make your DC2DC charging useless...
Do you know others?
and whats the best lead for DC2DC, in my opinion dirt cheap refillable FLA starter and always 2 smaller capacity then one big.. other opinions?

[markcouz]

Any decent DC2DC will have a switch on/switch off voltage [eg ON=13.2/OFF=12.8]. If you have a dead start FLA then it is unlikely to reach the 13.2 ON voltage required.

Even if it does, how is that different from any other boat that has a dead FLA and no DC2DC?

[CaptainRivet]

Originally Posted by markcouz

Any decent DC2DC will have a switch on/switch off voltage [eg ON=13.2/OFF=12.8]. If you have a dead start FLA then it is unlikely to reach the 13.2 ON voltage required.

Even if it does, how is that different from any other boat that has a dead FLA and no DC2DC?

But your alternator runs voltage gets ramped up and you get 14V+x in this Case and DC2DC will work.
It’s different because the DC2DC will draw its max amp eg 30A and dead lead trying to charge with the 30A=gassing=thermal runaway AGM while a dead AGM only the alternator won‘t deliver any to very less current. That’s what I experienced in a car, then ripping out the AGM on a highway parking.

[markcouz]

Surely if your FLA start battery was as dead as you suggest in this scenario then you would never have been able to start your engine and there would be no alternater running to melt your boat?

Why would any boat owner who has spent big bucks on LFP installation not just replace this dodgy FLA?

Note that the 30amps drawn by the DC2DC doesnt need to pass through the FLA first, it goes directly from alternater > DC2DC > LFP

[CaptainRivet]

Originally Posted by markcouz

Surely if your FLA start battery was as dead as you suggest in this scenario then you would never have been able to start your engine and there would be no alternater running to melt your boat?

Why would any boat owner who has spent big bucks on LFP installation not just replace this dodgy FLA?

Note that the 30amps drawn by the DC2DC doesnt need to pass through the FLA first, it goes directly from alternater > DC2DC > LFP

Well it happened to me, the BMW was 6 month old, so was the AGM and the Victron Orion DC2DC to an 40AH LTO pack for the car stereo. Car was standing 2 month (was my business car and standing at an airport parking) and didn‘t start. So I bridged LTO and started car, thought I left light on eg glovebox light…and was driving home when I suddenly had trouble breathing and smelled extrem gas (BMW Station Waggon and battery was in sidespace in cabin), stopped at a highway parking and figure out quickly the AGM was gasing, hot and started already deforming. so I used the emergency fire blanket to cover it and rip it out…reason I figured later was alternator regulator on a 6 month old BMW was faulty and AGM ran dry. If that would happen anyway without DC2DC I don‘t know or the DC2DC made it worse, that what I guess happened. A depleted battery acts like an additional consumer and tries to suck in the additional 30A coming from alternator for the DC2DC.
With boat:
Because you are in a eg remote area or it’s not starting so you just recognize the starter is depleted to 9V and you are at an anchorage and a storm comes and you need to leave…just 2 out of many reasons. so you eg started engine by joining the starter battery of the other hull (when it’s a cat) or house (mono). Aim startmthe engine because you need to leave and to also charge the starter. You think Starter is low because you were for 2month at the same anchorage…you don‘t think it has an internal short or the regulator is faulty and it ran dry before=>you cannot see if an AGM is dry.

[markcouz]

Ok I understand what happened in your case and why you are concerned - but this could happen to any lead acid/alternator combo, with or without a DC2DC installed.

The DC2DC is just a load, same as any other load in your system.

[CaptainRivet]

Originally Posted by markcouz

Ok I understand what happened in your case and why you are concerned - but this could happen to any lead acid/alternator combo, with or without a DC2DC installed.

The DC2DC is just a load, same as any other load in your system.

Question is if DC2DC made it worse...i guess yes because its starter on boat that has no other loads.
Just try to figure out if there are other cases and whats best to mitigate risks.
So far:
Thats why never an AGM as starter
Best solution so far use cheapest FLA you can refill, make sure vents are free and use 2 small instead one big. In above case it would just gas a bit but nothing else would happen. And you can refill if you need to run this combo in an emergency.

[Pete7]

Surely a failed lead acid starter battery will also cause this. One cell shorts out and the alternator sees 10v so pours power in resulting in the electrolite boiling off. As already explained, the engine doesn't normally start the engine so the big bang is avoided.

Sticking lot of power in from solar or shore power could be a problem.

Pete

[newhaul]

Originally Posted by Pete7

Surely a failed lead acid starter battery will also cause this. One cell shorts out and the alternator sees 10v so pours power in resulting in the electrolite boiling off. As already explained, the engine doesn't normally start the engine so the big bang is avoided.

Sticking lot of power in from solar or shore power could be a problem.

Pete

But a shorted cell would not allow the battery to start the engine either.
And the new generation of smart alternators iirc require a minimum of 10.5v or so to even start charging to protect from just such a failure situation .

[Pete7]

True and on a frosty morning when it won't start and needs a new battery, a melt down is probably avoided.

Pete

[markcouz]

Originally Posted by CaptainRivet

Question is if DC2DC made it worse...i guess yes because its starter on boat that has no other loads.
Just try to figure out if there are other cases and whats best to mitigate risks.
So far:
Thats why never an AGM as starter
Best solution so far use cheapest FLA you can refill, make sure vents are free and use 2 small instead one big. In above case it would just gas a bit but nothing else would happen. And you can refill if you need to run this combo in an emergency.

And the answer is no!

The DC2DC is a load on the alternater, not a load on the start battery - the DC2DC (unless over-ridden in programming) will only switch on when it sees a positive charge voltage (e.g. 13.2V), at this voltage the start battery is accepting charge, not discharging.

When the input voltage to DC2DC drops below the OFF setting (e.g. 12.9v) then the DC2DC will shut off to prevent draining the start battery.

You can see from the above that the DC2DC is a simple load, no different to an air conditioner, that is powered by the alternater and not by the start battery.

I am not sure why you keep circling around this issue like a hyena on a carcuss!

The problem in your case is that you ignored your start battery strong hint that it had failed by jumping it from the LTO battery - at that point you booted up your alternater which started pouring power into a failed battery causing the thermal runaway - the DC2DC had nothing to do with it and didnt make matters better or worse.

[newhaul]

Originally Posted by Pete7

True and on a frosty morning when it won't start and needs a new battery, a melt down is probably avoided.

Pete

A pair of U1 wheelchair SLA 35ah batteries would give about 650cca and are small enough to put about anywhere in the engine space. One by itself would do the job even on larger engines if paralleled with lifepo4 house for starting .

[Pete7]

Originally Posted by CaptainRivet

and whats the best lead for DC2DC, in my opinion dirt cheap refillable FLA starter and always 2 smaller capacity then one big.. other opinions?

Actually one of the problems this side of the pond is cheap FLA batteries are becoming extinct with sealed ruling the roost. Seems the manufacturers don't want you to top up the battery, instead pushing you to buy a new one instead.

More expensive Trojans and Rolls FLAs etc are still available which we are using in the hybrid house bank.

Starter is AGM badged Bosch.

Pete

[newhaul]

Originally Posted by Pete7

Actually one of the problems this side of the pond is cheap FLA batteries are becoming extinct with sealed ruling the roost. Seems the manufacturers don't want you to top up the battery, instead pushing you to buy a new one instead.

More expensive Trojans and Rolls FLAs etc are still available which we are using in the hybrid house bank.

Starter is AGM badged Bosch.

Pete

Actually even the "sealed" lead can be opened to add water just not easily done the first time. ( there are dozens of how to videos on YouTube)

It's also our disposable way people look at things these days.

[CaptainRivet]

Originally Posted by newhaul

Actually even the "sealed" lead can be opened to add water just not easily done the first time. ( there are dozens of how to videos on YouTube)

It's also our disposable way people look at things these days.

well sadly not, a lot are SLA glued close or have no access anymore to the single cells. But yes you can tagret to buy SLA which you can open eg most Varta can be opened by ripping off the top stick.