In the last chapter in this Online Book, we looked at battery switches and how to use them to build a cruising boat DC electrical system that properly separates the engine start and house banks.
And earlier we specified a third critical-load backup lead-acid bank to make lithium-based systems offshore capable and seamanlike.
So now we have all these batteries separated, how do we make sure they all get charged, particularly when the engine is running?
I have an article all written, that we will publish in a few days, analyzing six options for charging more than one bank of batteries at once.
But first let's cover good practice for battery bank separation and multi-bank charging:
I guess a lot of people would like to augment their capacity by adding lithium to an existing LA based system, but most advice assumes a complete changeover to all or mainly lithium. Would this work… (simplified re protection, monitoring, alarms etc.). The purpose is to add extra capacity in lithium to an existing LA system so that any risk of change is avoided as much as possible.
PV panels feed a 2 way and off switch. One way goes to MPPT chargers configured for LA and onto the existing common charge/load bus with the LA house bank on it. The other way goes to another MPPT charger configured for lithium going to prismatic cells with an external BMS. The lithium battery go through a DC to DC charger the LA with an isolator switch in that connection.
This assumes you have enough PV to recharge the LA everyday with some time to spare. One could add more LA, but that would probably mean more PV. With the spare PV time you can charge the lithium and keep it in reserve for a string of dull days. You could also have a basic inverter on the LI bank to use for non-critical 240VAC appliances when you didn’t need to keep the reserve.
One hole in this configuration might be the DC to DC converter. I’m told they run hot, are not very efficient and aren’t really suitable for ‘always-on’ use. (Hence the isolator mentioned above) I’m sure there are other challenges?
I don’t see how one could gain any significant benefits with that setup.
The main reasons for choosing LiFePO4 over lead-acid are:
If you have a large lithium house bank AND a large lead-acid house bank, both cycled regularly, then you have the worst of both worlds — all the complexity, cost, installation labour, protection circuitry, etc. of lithium, plus all the weight, monitoring labour, and maintenance of lead-acid.
Generally, in a LiFePO4 system, I like to see lead-acid cells used only for two specialized places:
DC-DC converters and DC-DC chargers are established, proven technology. The good ones are efficient and highly reliable in 24/7 multi-decade service. When one does fail, it takes only a few minutes to swap for a spare. They are not to be feared.
Thanks for the fielding that. I did not see your comment when I added mine. Yours is much better and more complete than mine.
There are certainly all kinds of ways we can mix this stuff up, but, at least to me, they are all kluges that just don’t make much sense. The key, and in fact only real benefit—now we have good solar to top up lead-acid banks—with lithium is higher energy density relative to weight and size. Given that I would not bother with these add on systems, but rather either do it right or stick with lead-acid.
I have written more about DC/DC chargers and isolators in the next chapter comming next week.
Matt, John, Your answers assume a blank canvas. I, and many others, have a well proven, already paid for and low risk LA based solution. I just want some extra capacity for a string of cloudy days without having to festoon the boat with PV. What’s the best way to do that?
I often here people talking about the risk associated with the technology, but this is nothing compared to the risk of change and the situational risk. ‘Situation’ in my case is …about to make an ocean passage to remote locations. So completely changing from a LA based system to all lithium is a non starter. Also, the failure rate of lithium vs LA is immaterial against the binary nature of failure of lithium and the lack of user intervention. Which one might not think a problem if your coastal cruising.
I see what you mean, but even so I would stick my my recommendation to do lithium right or not at all, and not try some sort of hybrid kluge.
Also, in the event of a string of cloudy days, you will still have to recharge a hybrid, or lithium bank, regardless of size. So a simpler answer might be to look at upping the size and efficiency of your alternator and regulator.
Of course whether or not this will solve your problem is usage profile dependant, but it is important to really analyze that option since I often see people throwing huge amounts of money at their electricity storage, when their actual problem is inadequate electrical generation.
And I totally agree that festooning a sailboat with solar is a bad idea.
“Extra capacity for a string of cloudy days” in a system that you’re otherwise quite happy with sounds like a generation vs. demand matching issue, not a storage issue. Reducing consumption, followed by a really good mounting & belt scheme for a really big alternator and a really good regulator, would be my go-to recommendation there.
Hi John, I agree with you on your comment, do it once, do it right. I think there is a strong argument for building a lithium in 48v. All the heavy lifting, charging, storage, inverting is done at 48v. Step down DC-DC for existing 12/24 loads are easily and cheaply achieved with only a slight loss. I understand the perils of YouTube but here is some kit from Balmar that looks worthy of further investigation. https://youtu.be/PPJcmr3dImo
In my view, 48 volts adds a bunch of complication for very little benefit over 24 volt, at least for most usage profiles. We have two full chapters on choosing the right base voltage for a given set of needs starting here: https://www.morganscloud.com/2020/12/10/should-your-boats-dc-electrical-system-be-12-or-24-volt-part-1/