Need help deciding on DC/DC Charging

[Traveler7]

Hi Sailors,

I have a 10.5 KW Thunderstruck Motor with 280 AH of AGM Batteries at 48v and a 100 AH Agm house battery. I do not have solar on yet, but I think I want to wire the solar to the 48v bank only and add a DC/DC charger for the house battery when I am off shore power. I do have a 3500 watt generator as well, but want to add the solar for versatility and also use the regen when I can- does anyone see a problem or have a recommendation on a DC/DC charger that can take the 48v down to 12v?

Thanks!

[misssherry]

https://shop.pkys.com/Victron-Energy...er_p_7116.html.

Take a look at the "tech tip" in the link from pkys. They suggest using a mppt solar controller in order to get a regulated charge profile. The victron DC to DC 48-12 controller puts out about 10-15 volts that can be selected in 0.2 increments. That would work power supply but not as a smart charger.

[s/v Jedi]

I recommend to use a 48-12V dc-dc converter set to 13.2V fixed output. Once this is setup, you can disconnect and remove the AGM battery.

Check out the 48/12 30A model: https://www.victronenergy.com/upload...50-400W-EN.pdf

If you require more than 30A power, you can connect a second one in parallel, which is recommended anyway for redundancy reasons.

Alternatively, you can keep the AGM battery to cope with peaks in power usage. This will be okay at 13.2V but it will shorten the battery lifespan.

[BjarneK]

Originally Posted by misssherry

https://shop.pkys.com/Victron-Energy...er_p_7116.html.

Take a look at the "tech tip" in the link from pkys. They suggest using a mppt solar controller in order to get a regulated charge profile.

Using an MPPT controller for this sounds like a great idea, but it only works if the 48 V battery and the 12 V battery has no other ground connections than through the MPPT (and that would be quite an unusual setup). The problem is that the Victron MPPTs measure the charging current through the ground and if there is an alternative ground route from the 48 V battery to the 12 V battery, the MPPT will not measure the current correctly and may overload. During testing, I had a 15 Amp MPPT deliver 20 amps due to this issue. See here for more info: https://www.cruisersforum.com/forums...ml#post3595300

[BjarneK]

Originally Posted by s/v Jedi

I recommend to use a 48-12V dc-dc converter set to 13.2V fixed output. Once this is setup, you can disconnect and remove the AGM battery.

Check out the 48/12 30A model: https://www.victronenergy.com/upload...50-400W-EN.pdf

If you require more than 30A power, you can connect a second one in parallel, which is recommended anyway for redundancy reasons.

Alternatively, you can keep the AGM battery to cope with peaks in power usage. This will be okay at 13.2V but it will shorten the battery lifespan.

Isolated DC-DC converters have some advantages, but one downside is that they are not as efficient as non-isolated converters. The above converter is listed as being 87 % efficient, so if it is delivering 30 A at 13 V, it will generate 30 A * 13 V * (1 - 0.87) = 50.7 W of heat. I have not tried the large converters like that one, but have had to add fans to cool down Victron's smaller converters.

Victron has a series of non-isolated high power DC-DC converters which are more efficient, but unfortunately they do not seem to have any 48 V versions.

[Traveler7]

Originally Posted by s/v Jedi

I recommend to use a 48-12V dc-dc converter set to 13.2V fixed output. Once this is setup, you can disconnect and remove the AGM battery.

Check out the 48/12 30A model: https://www.victronenergy.com/upload...50-400W-EN.pdf

If you require more than 30A power, you can connect a second one in parallel, which is recommended anyway for redundancy reasons.

Alternatively, you can keep the AGM battery to cope with peaks in power usage. This will be okay at 13.2V but it will shorten the battery lifespan.

I want to keep a separate house battery. So, there are no "smart" DC/DC chargers? Surprising. I could step down the 48v to 12v and then use a 12v DC/DC charger if there are "smart" ones of those....Thanks for the input!

[Traveler7]

Originally Posted by misssherry

https://shop.pkys.com/Victron-Energy...er_p_7116.html.

Take a look at the "tech tip" in the link from pkys. They suggest using a mppt solar controller in order to get a regulated charge profile. The victron DC to DC 48-12 controller puts out about 10-15 volts that can be selected in 0.2 increments. That would work power supply but not as a smart charger.

This is probably the correct answer. I have a V 75/15 on my camper van and like it. So I would get two Victron MPPT's - 1 for DC/DC and on for Panel to 48v bank, Sounds reliable to me. I could also the shore power charger for the house battery and only rely on the MPPT controller when at the dock. The best part is no part. Thanks

[s/v Jedi]

Originally Posted by Traveler7

I want to keep a separate house battery. So, there are no "smart" DC/DC chargers? Surprising. I could step down the 48v to 12v and then use a 12v DC/DC charger if there are "smart" ones of those....Thanks for the input!

I listed an option to keep the battery.

The MPPT controller gives problems like someone explained.

[s/v Jedi]

Originally Posted by BjarneK

Isolated DC-DC converters have some advantages, but one downside is that they are not as efficient as non-isolated converters. The above converter is listed as being 87 % efficient, so if it is delivering 30 A at 13 V, it will generate 30 A * 13 V * (1 - 0.87) = 50.7 W of heat. I have not tried the large converters like that one, but have had to add fans to cool down Victron's smaller converters.

Victron has a series of non-isolated high power DC-DC converters which are more efficient, but unfortunately they do not seem to have any 48 V versions.

Which is why I listed the isolated version

[Fuss]

i imagine that you only need the isolated version when you want to have a different negative from the source …is that right? I always had isolated but now i have my 24v and 12v negatives connected. anyone care to comment?

[Statistical]

Originally Posted by Fuss

i imagine that you only need the isolated version when you want to have a different negative from the source …is that right? I always had isolated but now i have my 24v and 12v negatives connected. anyone care to comment?

Correct. Isolated converter will produce an independent negative but if that negative is tied into a common negative bus it is pointless as the isolation is broken.

ABYC up to 48VDC (possibly 60VDC*) doesn't require isolated circuits even for electric propulsion. In fact they make no reference to using separate negative DC busses for each voltage as an option so I am not sure it is even allowed at <60VDC (although it should so maybe just an oversight).

Above 60VDC an isolated negative bus is required for the propulsion pack. So isolated converters would be needed to avoid breaking that isolation (i.e. 72 VDC to 12VDC isolated DC to DC converter).




*Not really clear if the 60V they reference is nominal or peak but 48VDC nominal should never peak above 60v so it is good either way. 60 VDC is ambiguous. 72 VDC definitely requires isolation.

[Traveler7]

Originally Posted by BjarneK

Using an MPPT controller for this sounds like a great idea, but it only works if the 48 V battery and the 12 V battery has no other ground connections than through the MPPT (and that would be quite an unusual setup). The problem is that the Victron MPPTs measure the charging current through the ground and if there is an alternative ground route from the 48 V battery to the 12 V battery, the MPPT will not measure the current correctly and may overload. During testing, I had a 15 Amp MPPT deliver 20 amps due to this issue. See here for more info: https://www.cruisersforum.com/forums...ml#post3595300

I believe it all has a common ground so I will explore this option carefully. Thanks for the advice.