Guest multi bank charger

C-Wolfe · Posted: Wed Apr 09, 2025 1:15 pm Post subject: Guest multi bank charger

I have a Guest 3 bank charger 5-5-10 that I assume is original equipment for my boat (2008) and does not support LiFepo4 battery charging. I upgraded my house bank to 2 Lifepo4 and start to AGM, I also have a DC-DC charger from each start battery charging the house bank. I do not feel the need to replace the built in charger anytime soon since I do not typically spent much time in harbor. I keep the boat park at my house and anchor for the night while I’m out on the water so no real advantage to have a nicer built in set up. So here is my question, can I use the 10 Amps on one of the start battery and connect both of the 5 amps on the other battery? Having 5 amps on each will be adequate to top off the start batteries but at 10 amps each, I could let the house bank getting charge from the DC-DC charger. I do have a portable charger for Lifepo4 that is easy enough to use for the house bank but plugging-in the boat through the shore power and flipping a switch is even easier.

thataway · Posted: Wed Apr 09, 2025 3:10 pm Post subject:

One of the beauties of LiFePO4 batteries is that they accept a battery charge far more rapidly than AGM or FLA batteries.

I have always used a dedicated Li battery charger--in the case of the c Dory it was the Multiplus 2000 watt inverter which has an 80 amp charger built in. I have several other set ups: one for the SUVs for frozen food / long trip food--and use a 10 amp Li Battery charger for that 100 amp battery. I have a 6 bank 100 amp battery, with solar panels, and if I need to top those off, I use the same 10 amp charger, although I have a number of other chargers, mostly for FLE. I do have one 30 amp charger which I have used which has an AGM profile.

The Guest charger could charge the Li bank, and probably would get it close to full charge, but you would want to take it off at that point. Almost all of the current LiFePO4 batteries have a good BMS internal system, which will prevent overcharging, which could be a danger. Even with my dedicated 10 amp charger, I only leave it on until the batteries are fully charged (which means topped of, because the self discharge rate is so low).

Which DC to DC charger are you using? What is the state of charge of the Li bank, when you are at the end of a voyage and get home? How are you monitoring the Li bank? What is the reason you went to LI (use of inverter, CPAC, etc?--more than just house loads?).

Most days run, even with only the 30 amps we were using from the Honda 150, (limited by DC to DC charger) we almost always had full batteries at the end of a day's run. We did use the 80 amp charger at times when we had not run the engine for a day. Our used for the Li was Inverter to run microwave and induction burner, plus two chest freezer/refrigerator units. We still had a single group 31 for the lights, navigation etc. However we had a swith which could change that to the Li battery bank (NOT parallel the two banks). Monitor with Victron monitor and the MFD for the Mulitplus inverter.

If we did not run the engine, then we used a Honda Generator to run the battery charger on the Multiplus inverter (usually combined with running the induction burner, or the microwave.

C-Wolfe · Posted: Wed Apr 09, 2025 4:18 pm Post subject:

Thanks Bob,
I moved all my batteries and related switches and load protection inside the cabinets, that required retiring my flooded start batteries, my house bank was 2 AGM group 31 that I’m now using for my start. I needed to purchase 2 new batteries for the house bank and I got 2 LiTime 100 A as it was no more expensive then good AGM.
I have Victron Tr smart 18 amps (X2) that I can adjust the parameters and monitor from an app on my phone, same goes with the LiTime batteries, I don’t have any other monitors other then a voltage display. I don’t really feel a need to monitor any better than that as I don’t have any big load for the most part. Once back from a trip, the batteries will only need to be topped off and I will have a few days to do that, so no need for a high load charger. I’m planning to use a portable charger for the lifepo4 and the Guest for the AGM, I was just curious if I could use the full 20 amps available from the Guest unit

ssobol · Posted: Wed Apr 09, 2025 6:15 pm Post subject: Re: Guest multi bank charger

[quote="C-Wolfe"]... So here is my question, can I use the 10 Amps on one of the start battery and connect both of the 5 amps on the other battery?.../quote]

You have to check the manual to see if this is ok. Some chargers all this, some may not. Another thing to check, is that if you only have one of the 5 amp outputs connected (and the other is unused), will the 5 amp charger put out 10 amps? Again, some chargers will.

C-Wolfe · Posted: Wed Apr 09, 2025 9:16 pm Post subject: Re: Guest multi bank charger

[quote="ssobol"]

WCF · Posted: Wed Apr 09, 2025 10:05 pm Post subject:

Other than what has already been discussed, your DC-DC likely does not have any temperature compensation/cut off. Do your LiFE4PO batteries have heaters or low\high temperature disconnect?

colbysmith · Posted: Wed Apr 09, 2025 10:46 pm Post subject:

I have two of those exact same Guest chargers. I replaced them with Mastervolt. (The first from my 22, the second from my 25). I now use those guest chargers in my shed and garage for charging my tractor or any other batteries. I have the two 5 amp posts tied together. (Same thing as hooking both to your battery to get a combined 10 amps.) It has worked fine that way.

C-Wolfe · Posted: Thu Apr 10, 2025 12:15 am Post subject:

C-Wolfe · Posted: Thu Apr 10, 2025 12:21 am Post subject:

colbysmith · Posted: Thu Apr 10, 2025 9:04 am Post subject:

That’s hooked together in series I believe for 24 vts. However I’ve got them tied together in parallel for 12 vts and it works just as well. I believe you only have one set of monitor lights anyway for the two 5 amp posts.

thataway · Posted: Thu Apr 10, 2025 10:50 am Post subject:

Colby is correct.

The manual is confusing, but for the 3 bank charger (2621) The diagram is for :

C-Wolfe · Posted: Thu Apr 10, 2025 12:41 pm Post subject:

thataway · Posted: Thu Apr 10, 2025 4:18 pm Post subject:

Hooking the two 5 amp outputs may not give 10 amps. It depends on how the circuit is designed. If you combine the two, it would be wise to monitor the current input from the combined 5 amp output. Many Digital Volt/amp meters have an input for up to 10 AMPS--often a separate plug in for the + (red) wire, In that case put the Digital volt meter in series with the charging lead, and read the input as it first comes on. The Guest is not a "smart" charger to the most recent standards.

C-Wolfe · Posted: Thu Apr 10, 2025 5:09 pm Post subject:

Thanks Bob, I do have a multimeter and will monitor at some point, but as of now, my start batteries were just topped off with a smart charger before putting them back in the boat. I probably won’t get a full 10 amps regardless.

C-Wolfe · Posted: Tue Apr 22, 2025 1:02 pm Post subject:

I did a bit of testing on the Guest charger, hooking up Both 5 amps double the charge to my battery. I was getting about 3.5 amps out of a single output wire and 7 amps once connect together. My battery was full but getting drain through the dc/dc charger, I might get a higher charge if battery is lower, although I was getting a full 10 amps on the other battery out of the 3rd output wire so it might be as good as it get, I can live with that for now.