DC to DC charger?

[Panhandler]

I recently installed solar panels and charge controllers to charge our large house bank. However, the generator start batteries (two group 31) and the engine start batteries (two 8D) are still only charged by the AC battery charger powered by the generator. What do I need to charge the engine and generator start banks from the house bank? Is it as simple as a voltage sensitive relay that connects a circuit between them when the house voltage is over 13.6? Or is there a "smart charger" for DC to DC? Any insight on the best way to do this?

Thanks!

 

[john61ct]

DCDC chargers are more expensive. That is only needed if a direct connection does not provide the right voltage, or if the target bank is expensive enough you want it to get a precise multi staged algorithm.

Otherwise a VSR/ACR is fine. That is, if you really don't think they'll be fine with existing charge sources - they don't actually have any greater or different loads now do they?

 

[LaBomba]

I would likely put a switch after the solar charge controllers to direct power to the gen and start batteries as required for a top up, which should not be too often if they are not being used.

 

[john61ct]

Then maybe just put in a traditional manual 1/2/Both switch.

Solar could be Common, 1 House bank, 2 Starter bank, Both combines, also allows self-jump cranking.

Many different designs possible.

https://forums.sailboatowners.com/index.php?threads/modified-2-bank-topology-switches-layouts.182986

 

[DavidM]

What isolates your engine alternators from the other engine's start battery or the house battery? Whatever provides that isolation can do the same for the solar output. In other words you should think about isolation globally and use that same device for your solar output. For example if one of your start batteries charges the house bank directly and a combiner/isolator/??? charges the starting battery when the voltage is right, then use that scheme and connect your solar input to the alternator's output so it will be treated the same.

I won't go any further with this line because it is too complicated, particularly with twin engines to make any sense over the internet, but I will talk a bit about isolation in general:

There are several ways:

Diode isolator- Bad because most cause a voltage drop.

Simple combiner- Yandina and others make simple voltage sensitive relays. Good, but lets the two combined batteries share the same voltage from the charging source. Some (not me) think that this is bad.

ACR, automatic charging relay- In general these are just an electronic version of the relay device above.

Balmar's Digital Duo Charge- This one and others like it (don't know of any, but I will bet that there are some) provide isolation like the ACR but also have internal circuitry that limits the voltage profile supplied to the starting battery (usually the secondary one in a combiner setup). FLA, AGM and Gel profiles are available.

So look at your solar output as another charging source that must be combined and isolated from the others. If you already have a working isolation system, then just hook your solar output to your house bank. That is the one that needs it the most and it would be rare that your starting batteries needed it. This is the simplest solution of all.

David

 

[FF]

Another solution might be an $18 RV merge solenoid .

Power it with a switch to join the solar as you see fit.

You decide when it operates.

****

RV solenoids are Completely seamless and reliable with a single engine and house bank.

The ACC position of the engine key switch causes the merge after engine start.

KISS

 

[ranger58sb]

However, the generator start batteries (two group 31) and the engine start batteries (two 8D) are still only charged by the AC battery charger powered by the generator. What do I need to charge the engine and generator start banks from the house bank?

Are the main and genset start batteries not also charged by alternators on their respective engines?

If so... how often do you run those engines? And what's the self-discharge rate of the batteries? And/or do you have a parallel switch (or two)? And/or could you carry/use jumper cables (assuming diesel boat) if an engine isn't starting because of batter state?

IOW, do you really need to add another charging system?

-Chris

 

[fryedaze]

We ditched a Proisocharge combiner for two ACRs. I am very pleased with them. They isolate the start batteries from the housebank if voltage drops to 12.75.

 

[Panhandler]

Thanks, all. I appreciate the ideas and input. Sounds like a 1/2/off switch would be the way to go.

In answer to a few questions, we run the generator almost every day for a bit so that's fine. We run the main engine every week to three weeks, just depending on how often we get to cruise. The bilge pumps draw of the engine start bank. I could move them to the house bank and have no draw but I'm not crazy about the possibility of leaving the boat for a very cloudy week and finding the house bank drawn down and not recharged by solar.

Chris, you bring up a good point. It is diesel, I do have jumper cables, and the generator start bank should always be fully charged.

 

[john61ct]

IMO the bilge pumps are a deep cycling use.

Note cranking duties are really very minor, almost a waste to carry extra weight and add complexity just for that if you already need/have deep cycling.

Consider for the future, creating a second "starter/reserve" dc bank, allow that to share in carrying house loads, just put an LVD on it to ensure never depleted to the point it will no longer crank the various engines.

Getting rid of multiple separate Starter batts dedicated to nothing but cranking.

 

[FlyWright]

I think it's pretty much inevitable that if you rely on manual switches to control charge sharing, you'll eventually leave them in the wrong position and will drag a smaller battery down from the shared load.

I like the Yandina Combiner/Blue Sea ACR solution mentioned by David above but it's important to charge the larger bank first and selectively combine the other batts when the primary bank has reached at or near the end of absorption. For example, I tap off my 660AH house bank to charge my G31 start battery. Since I typically return to my slip with a full start batt and a charging house bank, I typically leave the combiner switched off. If I'll be away from the boat for several weeks, I'll combine them.

Whenever the charge stops, the circuits are split so the loads don't get shared.

It works for me anyway...

 

[FF]

"For example, I tap off my 660AH house bank to charge my G31 start battery."

What purpose does that serve?

If the charging V is over 12.8 the start batt will just sit and not be using charge amps if its full.

The most difficult part of charging is remembering to UN merge the house and start batts.

For me turning the engine key off should be all thats required.

 

[FlyWright]

"For example, I tap off my 660AH house bank to charge my G31 start battery."

What purpose does that serve?

If the charging V is over 12.8 the start batt will just sit and not be using charge amps if its full.

The most difficult part of charging is remembering to UN merge the house and start batts.

For me turning the engine key off should be all thats required.

The combiner shares the house charge current with the start battery when selected on. This provides a float charge to the start battery from shore power in the slip. (single bank battery charger)

No need to "unmerge" when charging as the combiner does this automatically when the charge stops. It's all explained in great detail on the Yandina website.