I found one from Renogy, but back charging is not even an advertised feature, and even the Amazon page says nothing about it. But the Renogy charger is only 20 Amps and I’d like to find something more powerful, say a 40-60 Amp charger for the LifePO4 and a 4 Amp “trickle” charger for the AGM House battery, to keep it topped up when using navigation equipment, autopilot, night lighting, etc, and a Voltage Sensitive Relay (VSR) to keep the AGM starter battery topped up for use only by the engine.
Anyway, one device would be wonderful, and space is a big concern, and of course everything you install seems to need more space for fuses/breakers, etc. which has also become quite the challenge in such a small space.
Problem is it’s a small boat and I’m quickly running out of room. I had to make a custom box just to hold the LifePO4 battery in a locker, because there was literally no other place to put it. So something like the Renogy, but smaller and more powerful would be wonderful. Clark from Temptress makes a device which is small and will do the job, but he wants 500 Euros for it, which is outside my budget target for this project. Sorry, about 50 other projects need fixing on this boat
So does anyone know of a similar product or products by Victron or another manufacturer that could do this job? I am considering just putting a VSR between the LifePO4 and AGM House batteries but it seems general advice is against it. Maybe a VSR + Manual switch to control it? Cheap and small, sounds good at least but I don’t know.
Hi Janvi, thanks for your reply. So my AGM batteries are charged by a 12V AGM charger connected to the shore power, and when the engine runs, by a charger connected to the alternator. The Lifepo4 battery is only charged by the solar panel and Victron MPPT 75|15.
So can you please elaborate on how you might solve my 2 primary problems:
No sun for days means fridge which is only connected to the Lifepo4 goes warm, and I lose my inverter and laptop/mobile charging station.
Hours of sailing on autopilot, esp at night will run down my ‘House’ AGM battery.
So I would like to be able to charge my LifePO4 from engine and from shore power. And I would like to be able to charge my house AGM battery from excess solar power in the LifePO4. Hope that makes sense
Focus of your two posts/questions are pretty diffrent. Assume your problem:
In one vehicle/boat, there are two diffrent and independend grids. AGM with alternator and LiFePo with MPPT and shore power charger. Possibly the AGM/alternator came with your boat and the solar system with MPPT and LiFePo is bought later for having more power available. Both systems are insufficient for use as they are sometimes too small and run out of power, each one for its own reason. On the other hand, the sum of battery capacity and the sum of its diffrent chargers (alternator + grid charger + MPPT) are possibly sufficient to supply the sum of all required loads (fridge, autopilot, mobile, laptop, Starlink - to be continued.
Dealing with the sum of currents, both systems need to be tied together. This is a problem as there are two diffrent battery chemistries in use what shouldnt be connected in parallel for this reason. If autopilot is a critical load, I would consider to run this suplied by the LiFePo battery. A simple changeover switch only for the autopilot as load with selectable power sources will do the job. You could do the same for your laptop/mobile charger but I can hardly give any advice without knowing about the absolute comsumptions, battery sizes and habits to use.
Mainly it can be recommended to have only one battery system with the size of both previous AGM and LiFePo. If you replace the LiFePo by AGM then connect both systems in parallel and you are done. If you replace AGM by another LiFePo, you need to have another shore powered charger and the alternator needs to be modified to charge LiFePo. Normal 12 Volt alternators are about 50 Amp or 600-800 Watts but any further advice needs to take the complete installation in account. BTW: I kicked out my 12V/gas absorber fridge as this was a disaster of efficiency.
Therefore start with a inventory list of exact power consumptions of all relevant loads. Then assume the worst case situation and calculate the required battery size. Only based on this, any specialist might give a recommendation for system design and its available or additionally requried power sources.
Well that is interesting. So it is possible to have a 3 way toggle switch, with one power coming from the LifePO4 + terminal, one power coming from the AGM + terminal, and the negative to a common ground? If so, that can be applied a few different ways. For example I could put my fridge (my biggest power consumer onboard) on a toggle and switch to AGM/shore power when in marinas, thus allowing the solar to fully charge the LifePO4 unimpeded by the running fridge.
Also then I imagine it might be possible to do the same with the MPPT, and toggle it to charge the AGM batteries when needed, and toggle to the LifePO4 battery when needed.
This might be an easy cheap fix at least for the short term.
Wow that is almost as surprising as Renogy not advertising the feature in their product. I have seen the XS and considered it for a while, but it’s quite expensive and with thick wires/cables, and fuses that need to go somewhere, AND the need to still purchase an AGM trickle charger, the whole thing got put on hold for 1) budget (about 500 all in with parts mentioned) and 2) the very principle of not wanting FIVE chargers on my boat. Sorry it just seems a bit silly at a certain point I mean 1. Alternator charger, 2. Shore power charger, 3. MPPT charger, 4. DC to DC (XS) charger, and 5. LifePO4 to AGM trickle charger. I mean geez where does it end?! I already feel like all of this stuff should be replaceable with 2-3 devices max. I’m kind of surprised someone hasn’t made it yet.
So I guess it is promising that the XS might get this feature added soon. Still head scratching that they haven’t implemented it yet, and that the industry hasn’t simplified and condensed all these devices into a few powerful, flexible parts. But I’m just a consumer who doesn’t want FIVE battery chargers on his boat LOL, not an electrical engineer
Be aware you also have to change the charge characteristics what fits to your AGM and LiFePo battery sizes beside switching the power line. Final warning: Never switch off your alternator with running engine (=load dump).
Reading your post again, I found a couple more questions/comments
Simply adding another AGM and connecting it in parallel is now what I wish I had done. I had no idea the complexity and exponential expense of adding LifePO4 into the mix and I’m really kind of regretting it at this point. Anyway…
The 2x 100Ah AGMs that I had were more than enough for the normal boat operation. It was only the fridge and a medium power laptop that was killing my AGMs by dragging the SOC below 40% on a regular basis and action had to be taken to protect my rather expensive AGM batteries. So I added a 100 Euro solar panel and 130 Euro MPPT and it ballooned into a 700 Euro project with cables, fuses, breakers, switches, a custom made battery box, and Noa solar panel attachments. Already it was feeling out of hand and taking away from other projects on board that desperately need done, with respect to both time and money.
Now I’m invested, committed, and just trying to stop the bleeding, money and time wise, while there is still some summer left
Out of curiosity, what kind of fridge did you kick out? Another big project that ballooned was the insulation of the fridge and compressor cooling improvement with a new 12v 120mm PC computer fan. Every step of extra insulation in and around the fridge itself has made a big difference, and now rather than running 75% of the time to be just cold, it’s running about 30% of the time in summer and actually freezing in the bottom which I’m very happy with. It’s a Danfoss BD35F compressor fridge. More efficient now by far, but still my biggest onboard power consumer. The laptop is a somewhat distant second.
Now having said that I really must compliment the 160Ah LifePO4 as it runs the fridge for over a week (with no sun) instead of 3 days I was getting from the AGMs, and I’m not having to worry if my engine will start in the morning And with the solar onboard, a 180 watt panel, will sustain it indefinitely. But we had a whole week of thick clouds last week and my system found it’s limits. Due to weather I was also using the laptop more and after 5 days I needed another source of power.
Finally, I’m leaning hard toward your suggestion of putting a three way switch (LifePO4 or AGM or Off) after my MPPT as my budget solution. I didn’t know such a solution was possible as I was told to be paranoid about mixing LifePO4 and AGM, which I understand now from other conversations on this very helpful forum, that such a switch will not actually do. Mixing negative ground wires, not only OK but necessary, mixing positive wires very bad. Got it! Also as part of this solution I will run the solar to the AGMs to power the navigation equipment indefinitely while under sail, and switch over to LifePO4 charging when on the anchor. I’ll also add a similar switch to the fridge to run off of the AGM bank when in marina on shore power. So even during a cloudy week, the LifePO4 will still top up. Perfect
That leaves the starter AGM completely untouched, except for engine starting which is another goal I hadn’t mentioned.
BTW, what kind of fridge are you using/did you use again? I’m afraid I might have the bad one…
Then I think I’ll hold off on the XS for now, due to budgetary constraints anyway, until it has the two way charging. But good to know and keep an eye on thanks
Right. OK, so the order would be- switch off solar power, switch MPPT output to other battery, login with app and change charging profile to match new battery selection, switch on solar power. And possibly switch fridge to new battery as well.
Danfoss BD35 is a good choice. Here is what I kicked out. The price of all my fridges is lower than the Danfoss BD35 without fridge. After this, I did experiments using a Liebherr 230V AC fridge with a 12V inverter. That time, I also used 12V AGM and failed with the fridge. At hot temperatures, the pressure in the refrigerant circuit increases and causes a high inrush current if the AC induction motor starts. For 12V inverters, this makes inrush currents of several 100 Amps while AGM have much higher intrinsic resistance than LiFePo. Further, the new fridge designs gain their efficiency by thicker insulation. As a result, I got complaints by my girl friend as she couldnt close the door with her baking tray inside. At home, we have old Bosch fridge with very same outside size what fits the same baking tray without problem. The old Bosch uses R12 refrigerant and is still in good condition. R12 is no more produced. In case of a leak, R12 punches the ozon layer and shows high CO2 equivalent but efficiency is unbeatable. To solve the compressor inrush current, I changed my battery to LiFePo what has lowest intrinsic resistance at all SOC. Thereby DC grid changed from 12V to 48V using a MP2-3000 for fridge, coffee-dispenser and washing machine. This are the only 230V AC loads, everything else is 48V DC. For illumination I use the Meanwell constant current LDDH drivers and for Laptops and other 24VDC loads a Meanwell DC/DC step down converter whos efficiency is much better than step up designs. Step up is also required to charge 12V battery from other 12V battery, independend of the directions. Dont ask me for the money I spend for improving the installation and collecting these experiences.
Yeah I did a quick search on the BD35 compressor and half of the reason I originally started the insulation project was to save it hours of use so I wouldn’t have to replace it.
I also wrestled with insulating inside the fridge and did everything outside that I could first, because I didn’t want to lose capacity. But I’m pretty happy with the balance I’ve got now. BTW I think my fridge also ran originally on R12 but I have no idea what’s in it now, maybe butane? Anyway another problem it had was it was overcharged and iced up all the way to the compressor, so I let a little bit out at a time until it was only iced a little ways outside the fridge itself, and called it good. Hope I never have to refill it! Then I’ll need to somehow figure out what’s inside…
It sounds like your system now is pretty nice! A washing machine and coffee maker would be welcome additions to my (next and larger) boat For now I make due all right, with a 1500 watt inverter. And it is fun
learning about all this stuff and the options available.
I’ve heard about the step up and step down technologies and know it can be used to power a laptop for example without the inverter, which is supposed to be way more efficient. I’ll get there eventually, but have enough challenge for now. Thanks for sharing
And with the solar onboard, a 180 watt panel, will sustain it indefinitely. But we had a whole week of thick clouds last week and my system found it’s limits. Due to weather I was also using the laptop more and after 5 days I needed another source of power.