Can I short the input/output of DC-DC chargers?

[Ryban]

I'm working on a system configuration for my small sailboat that involves a LFP based house bank, and a LA based start battery. The LA battery is always connected to the output of the alternator and is used to start the engine. The LFP bank is normally connected to the boat DC distribution bus, solar chargers, and AC inverter/charger.

The way I've designed the system (so far) is such that:

• The LFP bank should receive sufficient solar energy to stay charged based on our needs and environment, most of the time
• The LFP bank CAN be, with a 1/2 switch, charged from the engine alternator via a DC-to-DC charger (normally this one is not selected)
• The LA bank CAN be, with the same 1/2 switch, charged from the LFP bank via a second DC-to-DC charger (normally this one is selected)
• The DC distribution bus can be powered by either the LFP bank or the LA bank by via a 1/2 switch (normally LFP)

In a perfect world:

• If the BMSes take the LFP batteries (2x) offline, for whatever reason, a signal triggers a remote battery switch to connect DC loads, with no interruption, to the LA bank -- this is primarily to avoid an issue like an autopilot disconnect, or other critical systems.

However, with the DC-to-DC chargers, I am shorting the input/output of one of them if the remote battery switch is triggered, and the bank selection switch is selected to the LFP bank (which is what it would be selected to for this to make any sense).

I can't seem to find a lot of info on whether this is acceptable or not, or if it would damage the chargers. Does anyone have any info, or better ideas?

Here's the layout as described above:


Here are the shorted paths, depending on switch position:

 

[chrisski]

No good.

If I’m reading this right, you’re shorting the lithium bank directly to the lead bank when you only want one to charge it.

This could set up for a current rush if the AGM and lithium are at different voltage.

 

[Ryban]

No I don't think I'd be shorting the banks themselves. The remote battery switch would only trigger if BOTH LFP banks were disconnected. When the LFP banks are back online, the remote battery switch disconnects the LA bank.

If the manual 1/2 bank switch is turned to the LA position, then the LFP bank is also disconnected from distribution. The only paths between the LA bank and LFP bank when the BMSes are operating normally is through the DC-to-DC chargers.

 

[JayArr]

Personally I think you are over engineering this and that is part of why it is getting complicated.

Once that remote battery unit contacts you'll be in a loop that will just quickly run the battery down. Sort of like a snake eating it's own tail.

I'm coming from the RV world but Marine is very similar.

I don't think you need the DC-DC to charge the start battery from solar. A temporary boost switch should suffice there. This is done in RVs all the time, if the RV won't start you hold the boost switch down to connect the house batteries temporarily to the start battery only long enough to get it running.
The idea that you need to use the extra capacity from the solar panels to keep the start battery 'topped up' is admirable but unnecessary for it's function. That start battery should hold a charge for a month or more and still start the engine, (if not get a new one). Most people, if they are going to be away from the RV/boat for more than a month remove the battery and store it somewhere with a trickle charge on it.

The DC charger from the alternator to the house batteries is also a poor choice. That is a very inefficient way to charge the batteries. It looks like you wanted a failsafe, 'if you couldn't get enough solar you could start the engine' but that's not free, every kilowatt of energy supplied by the alternator to the house bank costs you $$ in gasoline or diesel. Don't think there is any 'free' power there, the alternator has a regulator and it outputs only what's needed, nothing is 'wasted'. If you increase the demand on the alternator then the engine consumes more fuel, simple as that. As a failsafe - warming up a large marine engine to run a 150A alternator is something that I would only do as a last resort.

IMO I think you should consider that you would be better off selling the two DC-DC converters and buying a small Honda genny. Hook it's AC output to the Inverter/Charger and let it charge the house batteries when there is insufficient solar. The Inverter/Charger likely has a much better charger in it with three or four stage charging etc. It's currently not being utilized. The little Honda would be much more efficient than charging your house batteries from your engine alternator.

If you really want to keep the double inverters you will need to come up with a switching network using big relays or contactors that disconnect the DC-DC converters when the remote battery contactor closes. This can be done but will be more work and possibly the same money as a small portable genny.

 

[Orris_Tx]

Personally I think you are over engineering this and that is part of why it is getting complicated.

Once that remote battery unit contacts you'll be in a loop that will just quickly run the battery down. Sort of like a snake eating it's own tail.

I'm coming from the RV world but Marine is very similar.

I don't think you need the DC-DC to charge the start battery from solar. A temporary boost switch should suffice there. This is done in RVs all the time, if the RV won't start you hold the boost switch down to connect the house batteries temporarily to the start battery only long enough to get it running.
The idea that you need to use the extra capacity from the solar panels to keep the start battery 'topped up' is admirable but unnecessary for it's function. That start battery should hold a charge for a month or more and still start the engine, (if not get a new one). Most people, if they are going to be away from the RV/boat for more than a month remove the battery and store it somewhere with a trickle charge on it.

The DC charger from the alternator to the house batteries is also a poor choice. That is a very inefficient way to charge the batteries. It looks like you wanted a failsafe, 'if you couldn't get enough solar you could start the engine' but that's not free, every kilowatt of energy supplied by the alternator to the house bank costs you $$ in gasoline or diesel. Don't think there is any 'free' power there, the alternator has a regulator and it outputs only what's needed, nothing is 'wasted'. If you increase the demand on the alternator then the engine consumes more fuel, simple as that. As a failsafe - warming up a large marine engine to run a 150A alternator is something that I would only do as a last resort.

IMO I think you should consider that you would be better off selling the two DC-DC converters and buying a small Honda genny. Hook it's AC output to the Inverter/Charger and let it charge the house batteries when there is insufficient solar. The Inverter/Charger likely has a much better charger in it with three or four stage charging etc. It's currently not being utilized. The little Honda would be much more efficient than charging your house batteries from your engine alternator.

If you really want to keep the double inverters you will need to come up with a switching network using big relays or contactors that disconnect the DC-DC converters when the remote battery contactor closes. This can be done but will be more work and possibly the same money as a small portable genny.

I may not be fully understanding what the full intent of his system is, but the Renogy DCC50S MPPT Charger/Controller will charge the starting battery from solar when the house batteries are full.