Charging options for boat Lithium house bank and AGM engine bank

Atomvoyager

New Member
I'm installing 220w of 12v solar on a boat with 200AH of 12v lithiums for house and 100AH AGM for engine. I want the two banks completely isolated for simplicity and fail-safety, not using a 1-2-off-Both switch because in Both position it could cause problems with different battery chemistries. I'll have two separate on/off switches instead. I plan to have the engine alternator output to the AGM with a Renogy 40A battery to battery charger to charge the Lithium bank when engine is on. That way the engine alternator is protected from a BMS cutout failure.

All other charge sources (MPPT solar controller and stand alone 10A mains battery charger) will go only to the lithiums, as well as all loads other than engine starter (switch panel and stand-alone 1000W pure sine wave inverter).

My question is what is the best way in this layout to solar charge the AGM to deal with self-discharge and ensure it is always topped for engine starting? I see the following options:
1. Install a dual output Lithium compatible MPPT to both banks. The only one I found so far is the expensive and big Xantrex 30A #710-3024-01. Together with the remote display it's $245 and is overkill at 30A. A 20A version that's less expensive and smaller size would be more appropriate. Does anyone know of another option in lithium dual output MPPT?
2. Perhaps I could run the MPPT instead to the AGM bank and and have a separate switch to energize the battery to battery charger but that seems inefficient.
3. Or use a second battery to battery charger from lithiums to AGM. Again that seems inefficient.
4. Another option is to use my existing Epever MPPT to lithiums only and ignore the problem and make sure to start the engine at least once a month to ensure it stays charged. In the rare event it became too discharged to start the engine then the wires from solar, shore charger, or lithiums could be transferred and used to start the engine. That seems least desirable and troublesome if the boat was left unattended for several months and the AGM self-discharged.
5. Or plugging in a 5-10A shore power charger into the invertor to charge the AGM from the Lithium. Probably very inefficient but doable on those rare times it's needed.
We're setting up the boat for live aboard mainly on the hook and only occasionally in the marina with shore power. I'm familiar with solar and 12v marine wiring but have no experience in mixed battery banks so would appreciate any recommendations.
 
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willo

Solar Addict
You really just want a maintenance charge on the AGM. This is easy enough.

Just add a small DC-DC charge controller that's disabled when the engine is running. This would be very easy with say, a victron 12/12. They have remote enable inputs that you could relay off your engine control or simply attach to a toggle. I like the smart versions as I can configure them with my phone.

Why this route? Your lithium is very efficient at sucking up power from the solar. AGM, not so much. Also, you could run a pair of victrons on a DPST toggle and flip a switch for whatever direction you want to charge. In a pinch, you could swap wires around if you somehow kill one.
 

Atomvoyager

New Member
Good ideas, thanks.
I think I'll get the Victron 12/12 9 amp converter and wire it for input from the lithium house bank and output to the AGM engine bank with an on/off switch to activate it as needed. We just need to remember not to have the other converter (The Renogy 40A) on at the same time going in the opposite direction. Not sure if that would matter or not though.
 

Fusion is the future

Solar Enthusiast
I'm installing 220w of 12v solar on a boat with 200AH of 12v lithiums for house and 100AH AGM for engine. I want the two banks completely isolated for simplicity and fail-safety, not using a 1-2-off-Both switch because in Both position it could cause problems with different battery chemistries. I'll have two separate on/off switches instead. I plan to have the engine alternator output to the AGM with a Renogy 40A battery to battery charger to charge the Lithium bank when engine is on. That way the engine alternator is protected from a BMS cutout failure.

All other charge sources (MPPT solar controller and stand alone 10A mains battery charger) will go only to the lithiums, as well as all loads other than engine starter (switch panel and stand-alone 1000W pure sine wave inverter).

My question is what is the best way in this layout to solar charge the AGM to deal with self-discharge and ensure it is always topped for engine starting? I see the following options:
1. Install a dual output Lithium compatible MPPT to both banks. The only one I found so far is the expensive and big Xantrex 30A #710-3024-01. Together with the remote display it's $245 and is overkill at 30A. A 20A version that's less expensive and smaller size would be more appropriate. Does anyone know of another option in lithium dual output MPPT?
2. Perhaps I could run the MPPT instead to the AGM bank and and have a separate switch to energize the battery to battery charger but that seems inefficient.
3. Or use a second battery to battery charger from lithiums to AGM. Again that seems inefficient.
4. Another option is to use my existing Epever MPPT to lithiums only and ignore the problem and make sure to start the engine at least once a month to ensure it stays charged. In the rare event it became too discharged to start the engine then the wires from solar, shore charger, or lithiums could be transferred and used to start the engine. That seems least desirable and troublesome if the boat was left unattended for several months and the AGM self-discharged.
5. Or plugging in a 5-10A shore power charger into the invertor to charge the AGM from the Lithium. Probably very inefficient but doable on those rare times it's needed.
We're setting up the boat for live aboard mainly on the hook and only occasionally in the marina with shore power. I'm familiar with solar and 12v marine wiring but have no experience in mixed battery banks so would appreciate any recommendations.
Change out the engine alternator for a Balmar with alternator protection and ready for lithium ( Balmar charger controller protector ) adjust at max charging rate for you battery bank
Keep the starting battery up with a good 3 step charger feed from your inverter, the in efficiency is not that big due a way better Alternator, and a good inverter has an efficiency of around 96 % . and if the starting battery is fully charged the power usage is small.
Each solar panel needs his own charger controller feeding the Lithium house bank.
A galvanic isolation from shore power is A must .
200 A/H seems a bit small for a live a board sail boat .
a small generator ( mobile) would give you an extra charging possibility and redundancy .
Calculate you power usage under sail for 24 hours without charging time multiply that with a factor 2 and you are good to go
(radar, agis , plotter/gps and radio on .
 
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Atomvoyager

New Member
Thanks for the info. At some point we will upgrade to the Balmar you recommended and will add a galvanic isolator on shore power.
Good to know the occasional use of a battery charger on the inverter from the house bank is not a problem.
200AH is smaller than typical but we don't have big loads - no fridge, watermaker, water heater or pumps other than small bilge pump. There is windvane steering with only light use of a tillerpilot. We can add another lithium if we add a fridge later. At anchor we expect to use about 30AH a day and up to 90AH offshore, although we can reduce that as needed by only running the radar and plotter a few minutes at a time.
 

Fusion is the future

Solar Enthusiast
Thanks for the info. At some point we will upgrade to the Balmar you recommended and will add a galvanic isolator on shore power.
Good to know the occasional use of a battery charger on the inverter from the house bank is not a problem.
200AH is smaller than typical but we don't have big loads - no fridge, watermaker, water heater or pumps other than small bilge pump. There is windvane steering with only light use of a tillerpilot. We can add another lithium if we add a fridge later. At anchor we expect to use about 30AH a day and up to 90AH offshore, although we can reduce that as needed by only running the radar and plotter a few minutes at a time.
Buy another good bilge pump, one in NOT enough ,and keep you bilge spotless clean, it is no fun when you have to repair a bilge pump while rock and rolling, i have done that under a running engine , the V belt was eating my hands, no fun at all .(Golfo de Papagayo 50 knots sustainable for 40 hours )
Make on a handheld GPS an emergency safe (escape)route in, in case you do not have power left, it is a fall back safety.
And do not forget to give every solar panel his own charger controller .
Check if Balmar has a 2 bank output option on his alternators, controllers .
IF your inverter fail and you have to charge the stating battery you can rewire the output from the solar panel charger inverters to the starting battery .
A change over switch from the charger inverters for house bank and starting battery is also not a bad idea.
You can easy charge an AGM battery with an Lithium charge curve (till 80% topping of is not possible due the lower max V )NOT the other way around.
You 200 A/H bank is in reality 160 A/H lithium does not like to be discharge till 0 20% is a good figure for the life span .
Check the c20 rate,it is mostly the advertised one .
The max charging current is an important factor for sizing your new alternator choose a bigger alternator than you need now and limit the current in his charger controller, you can later add more lithium and your alternator, belt life span increase, if i remember well Balmar has also a soft start.
I advise also an AGIS transmitter receiver a stand alone with interface on your plotter.
VHF set , one base station and one hand held is a must .
A lot of countries let you download their maritime charts for free , USA , CANADA , BRAZIL i know for sure.
Get yourself a fully enclosed inverter NO VENTILATOR type, blowing salty humid air trough your electronics is realy a bad idea, and advise FOR ALL elctronics on a sail boat.
A mini pc fully enclosed with a monitor is so much cheaper than a plotter, and you can interface it with any thing you want .
Run Predict Wind on your mini PC and you can download for free weather maps and forecast for 5 days !!
Instal a WIFI extender .
 
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noenegdod

Solar Addict
This is how Im doing it. Its a 300A diode so kind of overkill for what your doing but a diode that can handle 5 or 10 amps I would assume would be sufficient. You would just have to isolate before cranking with a simple relay connected to the starter.

I went with 300A so I can have a very small AGM battery for the charging system but when cranking the Li battery will do most of the work.

Forward voltage on this IIRC is 0.36v volts at 300A and at 950amps its 1.5v


20210805_213143.jpg
 

Fusion is the future

Solar Enthusiast
This is how Im doing it. Its a 300A diode so kind of overkill for what your doing but a diode that can handle 5 or 10 amps I would assume would be sufficient. You would just have to isolate before cranking with a simple relay connected to the starter.

I went with 300A so I can have a very small AGM battery for the charging system but when cranking the Li battery will do most of the work.

Forward voltage on this IIRC is 0.36v volts at 300A and at 950amps its 1.5v


View attachment 64255
calculate your lost . .
 

syclonedriver

Solar Enthusiast
There are some very knowledgeable folks posting here.

I want to offer up another option for charging your start battery. Find room to one more panel, about 20 watts and use a separate MPPT controller on it. You could use one just like you are using for the larger panels, that way if one ever failed on the main system you would have a backup.

I have added refrigeration to two boats, my current boat had it factory installed. I love not having to stop for ice or having to pack a cooler. I'm not living on my boat, that's years away. I have a 2008 Beneteau Oceanis 31. What have you got?
 

noenegdod

Solar Addict
calculate your lost . .
Context is important to consider.

You edited our post but the example in the notification I received was 10a X .7v = 7 watts. If the boat system was consuming 10 amps continuously when the key is off that needs to be fixed.

The op is concerned about battery self discharge and the battery discharging over the course of months, not about trying to supply a significant amount of current.

You would never charge through a diode if you needed to supply large currents for an extended period. You would never use a diode if you needed to supply even modest current like 10 amps constantly.

How much current is required to keep an AGM charged? Calculate your loss on that.
 

Atomvoyager

New Member
Thanks for the many good and appreciated points here. Many options to choose from. This sailboat is an Albin Ballad 30 that's currently in my backyard boatshed. I'm refitting it for a friend who is not skilled in solar or electrics and he's relying on my advice as we go forward. My own boat is an old Pearson Triton 28 that I've sailed and lived aboard extensively, although in a more minimalist mode so some of this lithium related electronics is new to me. I did recently add a single 100AH lithium battery and two solar panels to my other boat, a cruising version of an F-24 Corsair trimaran but that was dead simple with no inboard engine and minimal electrics involved.
 

Fusion is the future

Solar Enthusiast
Context is important to consider.

You edited our post but the example in the notification I received was 10a X .7v = 7 watts. If the boat system was consuming 10 amps continuously when the key is off that needs to be fixed.

The op is concerned about battery self discharge and the battery discharging over the course of months, not about trying to supply a significant amount of current.

You would never charge through a diode if you needed to supply large currents for an extended period. You would never use a diode if you needed to supply even modest current like 10 amps constantly.

How much current is required to keep an AGM charged? Calculate your loss on that.
Self discharge of a good AGM is around 1 a 2 % a month and charging trough a diode from a house bank you never get the AGM fuly charged ! https://www.doityourself.com/forum/...charging-agm-batteries-battery-chargers-2.jpg
 

noenegdod

Solar Addict
Self discharge of a good AGM is around 1 a 2 % a month and charging trough a diode from a house bank you never get the AGM fuly charged ! https://www.doityourself.com/forum/...charging-agm-batteries-battery-chargers-2.jpg
The forward voltage makes sure of that. It will always be .3 - .7 volts lower than the house battery. Thats not the point. The OP wants to make sure the AGM is never dead. If the solar is charging the house batteries to 14 volts the AGM will see13.3 to 13.7 volts.

It will never be dead.
 

sailingcal21

New Member
The forward voltage makes sure of that. It will always be .3 - .7 volts lower than the house battery. Thats not the point. The OP wants to make sure the AGM is never dead. If the solar is charging the house batteries to 14 volts the AGM will see13.3 to 13.7 volts.

It will never be dead.
Indeed it will never be dead, however if not brought to 100% SOC, from a capacity prespective it will have a slow death.

I concur with syclonedriver 's comment to add a second smaller PV system for the AGM. You'd also have a more fault tolerant system.
 

noenegdod

Solar Addict
Indeed it will never be dead, however if not brought to 100% SOC, from a capacity prespective it will have a slow death.

I concur with syclonedriver 's comment to add a second smaller PV system for the AGM. You'd also have a more fault tolerant system.
There is no question there are better ways to accomplish the task from different perspectives however there is also no question there that there is no simpler, more reliable or cheaper ways to get the job done. If the boat gets used every few months the battery will get charged to 100% then. In the mean time, when the operator shows up to use the boat, the battery will start it.
 

syclonedriver

Solar Enthusiast
There is no question there are better ways to accomplish the task from different perspectives however there is also no question there that there is no simpler, more reliable or cheaper ways to get the job done. If the boat gets used every few months the battery will get charged to 100% then. In the mean time, when the operator shows up to use the boat, the battery will start it.
I do like the simplicity of the diode!
 

DJSmiley

Solar Addict
There are multiple MPPT's which do support a starter battery.

Eg the Epever Duoracer has a seperate output for a lead-acid starter battery

or the Votronic MPP 250 Duo

This allows you to charge the starter battery to a higher voltage than the LFP bank. If you set the LFP bank to eg 14.2V and add a diode, the starter battery is charged even lower, but it's generally recommended to keep a lead-acid at a higher voltage.
 

Fusion is the future

Solar Enthusiast
It is stupid simple. There are obvious drawbacks too. I think your suggestion above using a separate solar panel and controller is probably the best option if there is room for the panel and cost isnt an issue.
I do not like it,if the start battery get a plate shortage, you drain the house bank with a high Amperage .
 

noenegdod

Solar Addict
I do not like it,if the start battery get a plate shortage, you drain the house bank with a high Amperage .
That's why you use a small diode and use a DPDT relay wired to the starter. The diode is connected to the AGM through the NC contacts in the relay. When you are starting, the contacts open and the diode sees no current. When done cranking the relay returns to the NC position but the alternator is producing so voltage should be higher than the house so no current coming from house system.

If there is a short in the AGM system and the diode sees high current it will burn out. You could also stick a fuse in the line if you were afraid the diode might dead short instead of opening.

I am using a very large diode because I am using a very small AGM starter battery that is just adequate to start the engine on my truck. If it is hard starting I want the house system to be able to supply large amounts of current to continue trying. Yes, I have done the math and there is about 220 watts of heat being generated in the diode while cranking which is exactly why it is (will be) bolted to a large copper bus bar to help act as a heat sink. I will also have to be aware of how long I am cranking if I am having issues and monitor the diode.

I am using a diode here because there is a Victron battery protect up line from the diode. I thought that the BP only allowed current one way through it. Later on I found out that it must not have current go backwards through it. It will allow current both ways through it but if it travels backwards through it it will kill it. Hence the diode. I would have been very happy to have not needed it.

I had considered just using a switch or contactor if I needed to supplement the vehicle system but once the alternator starts producing power, current will travel backwards through the BPs and kill them. A diode was the simplest and most reliable solution.
 
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