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Options for LVD protection for an inverter

Aphers

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Nov 17, 2020
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I'm planning on a 12v 280Ah system, and want to run some 240v equipment- the most demanding being a 1600w induction hob. So I'm planning on running a 2000w PSW inverter at least. Ideally I'd like to be able to run as much 240v as the batteries can safely take, but 1600w is the bare minimum.

I'm trying to work out the most cost effective way of hooking up the inverter. In no particular order, and not necessarily mutually exclusive, I think I can:
- have a suitably beefy BMS with the current running through it, e.g. 250A
- use a smaller BMS to trip a beefy relay cutting all of the loads- possibly a SSR (cheap and nasty?)
- use the BMS to trip the inverter itself, if it can do that. Or hack the switch on it if not.
- use the inverter's inbuilt voltage detection (yes i know this is not cell level, so it would just be a secondary layer of protection)
- fit a device on the 240V side of the inverter, still triggered by the BMS, to cut the output- could be nice and cheap because of much lower current- not sure what this would look like but it sounds promising.

I could also fit a couple of inverters, say a 2kw and a 1.5kw, which would make the individual relays cheaper but also makes things a lot less flexible.

So which of the above options would be best? I am hoping to use a pretty cheap inverter but I could be persuaded to upgrade that to something that has clever protection features that save me buying big expensive relays.

Thanks for any suggestions...it's easy to get overwhelmed with these decisions.
 
I'm planning on a 12v 280Ah system, and want to run some 240v equipment- the most demanding being a 1600w induction hob. So I'm planning on running a 2000w PSW inverter at least. Ideally I'd like to be able to run as much 240v as the batteries can safely take, but 1600w is the bare minimum.

I'm trying to work out the most cost effective way of hooking up the inverter. In no particular order, and not necessarily mutually exclusive, I think I can:
- have a suitably beefy BMS with the current running through it, e.g. 250A
- use a smaller BMS to trip a beefy relay cutting all of the loads- possibly a SSR (cheap and nasty?)
- use the BMS to trip the inverter itself, if it can do that. Or hack the switch on it if not.
- use the inverter's inbuilt voltage detection (yes i know this is not cell level, so it would just be a secondary layer of protection)
- fit a device on the 240V side of the inverter, still triggered by the BMS, to cut the output- could be nice and cheap because of much lower current- not sure what this would look like but it sounds promising.

I could also fit a couple of inverters, say a 2kw and a 1.5kw, which would make the individual relays cheaper but also makes things a lot less flexible.

So which of the above options would be best? I am hoping to use a pretty cheap inverter but I could be persuaded to upgrade that to something that has clever protection features that save me buying big expensive relays.

Thanks for any suggestions...it's easy to get overwhelmed with these decisions.

You've laid out all the options. It's pretty much up to you. 24V would make your life a lot easier. You're going to have to balance complexity, price and function.

A comment: low voltage disconnect of the inverter is the PRIMARY layer of protection. It's where you decide to establish your operational limits within the protection envelope of the BMS. The other end is selected via your peak charge voltage. the BMS is secondary as you never want to utilize it routinely - it's the safety behind the scenes. Inverters and chargers that can be programmed to change the LVD are desireable.

Have you conducted an energy audit to establish your needs to design your system (link #1 in my sig)?
 
Thanks snooblers, I do appreciate your help. I ran the numbers on the energy audit and I don't think I'm too far off the mark. It suggested just over 1kw PV, which I can't quite fit on the boat. I'm planning 800w, I could squeeze in another 300w I suppose but it would be in shade a lot of the time. The suggested battery capacity was 360Ah, I am tempted to stick with 280Ah for simplicity/cost as the cells are readily available and good value. I am keeping my old lead-acids as well (3x110Ah) so they get me over the line... except they will be part of a completely separate system so I can't really count them for this purpose.
One thing that is hard to convey in an audit is that we plan to be pretty flexible with our energy use. On days when solar power is short, we will switch back to cooking with gas, which will chop at least a third off our electrical consumption. I think that if we do that, our 800w+280Ah should be sufficient.

Choosing the inverter seems a bit tricky. One the one hand there are lots of generic cheap ones around, sub £300 for 2kw. The Renogy one fits this bracket but at least has a known brand name on it. Those that bother to give specs all have a 10.5v alarm and a 10v shutdown. I have also come across a Xantrex model which has these protections half a volt higher (11v alarm, 10.5v shutdown).Marginally better but not really worth the £360 price tag for that feature alone.
Double that price again and you can get a Victron 2KVA, which I presume will do the job- although only just, as the continuous rating of 1600w is right on the lower limit of what I need.

Some of the cheaper inverters offer a remote switch panel, which at a glance suggest a way of shutting down without having to handle the full 170A current. But of course that would depend on the BMS to activate it, unless some additional voltage sensing device is fitted.

I could go 24v. It's very tempting. The LFP system will be entirely new and apart from the cabin lighting the main loads on it (inverter and fridge) are yet to be purchased. Likewise the charging system (MPPT+PV) has not been bought either. But it strikes me as a real concession to have to make. I like the flexibility and redundancy of being able to swap loads from one system to another as the system evolves. There is even the possibility that I decide my budget just cannot stretch to LFP- or I cheap out too much and end up ruining my cells- and I end up with a bunch of useless 24v hardware that won't work with my old 12v system.
 
Okay. Boat and UK. Power needs are 230VAC/50Hz?

You sound very budget constrained. The reality is that even a dumb BMS is sufficient to protect your cells from damage, but they're not going to restrict the operating range.

If you absolutely must have the desired functionality, rather than try to devise an elaborate work around, in contrast to my prior comments, probably best to invest in a smart BMS that's programmable. While it's preferred to use the inverter/charger to set those limits, it's often cheaper to use the BMS. There are Daly Smart BMS that are reasonably priced (not cheap) and can handle the current.
 
I could also fit a couple of inverters, say a 2kw and a 1.5kw
Keep it simple!
A 3000w inverter with high and low V cutout. Do any come without that these days??
Stock standard minimum 250amp rating up to that.
 
@snoobler yes we're UK based (for now) so 230v. Plan to leave next year on an indefinite cruise.
The budget can be somewhat flexible, but there is an entire boat to kit out and lots of priorities to balance. My default position is £700 for a bank of 4 Trojan deep cycle batteries, but I think I may be able to install a DIY LFP system for not much more- and realise some significant benefits, including partially electrifying the galley.

I'm very much willing to make some choices to trade convenience for cost- that's why the LFP system would be entirely separate from the existing FLA one. So I don't need to worry about abrupt shutdown, because it's not powering anything critical. I don't need to worry about my alternator, because it's only charged by PV.

If I go down the route of a Daly smart BMS, which seems very reasonably priced at about £100, should I pair it with a latching relay, or does that not really add any further protection since it still relies on the BMS?
 
I've looked in to the Daly units and some of the reviews/comments aren't entirely favourable, e.g. failing fans and inability to deliver the claimed current. I guess I could generously over-size and go for a 300A unit? Or accept that I'm cheaping out and taking a risk with my cells :eek:
I'm hearing good things about Overkill and REC but finding them harder to source, or not available with the kind of power handling I require.

By the way I've found out that the fridge compressor I plan to install has a LVD at 10.6v, which is nice to know.
 
Keep it simple!
A 3000w inverter with high and low V cutout. Do any come without that these days??
Stock standard minimum 250amp rating up to that.
Thanks. Even the really cheap units tend to have these, but the preset values are a bit wide, especially LVD which is usually just 10v. Mind you I'm finding the LVD for the Daly might be the same value so it would be a toss-up which one would kick in first.

A Victron battery protect has a 12v preset which would be a big improvement, but the more I read about these devices the more fussy they sound about how they are installed.

Perhaps it's a bit paranoid but I am thinking it would do no harm to wire up a mini voltmeter to each cell for instant real-time info. I am used to watching the volts on my lead-acids like a hawk whilst using them (no LVD devices there!) so it would seem natural to do the same on the new bank. And the big loads will only be drawing whilst someone is aboard and awake.
 
OK so I've stumbled across a 3kw inverter that is nicely in budget and has a LV alarm at 11.5v, and shutdown at "10-10.5v" which is a bit ambiguous but no worse, potentially, than the others in this price range.

I think I may have found my cells supplier. Next up, BMS- Daly being the obvious choice for a tightwad like me. The version with fan cooling would cost me £108 for 250A, or £158A for 300A which gives me a bit of headroom- I guess that's very advisable? These prices will be plus taxes and duty so in reality I'm looking at nearly £200. There might be better options than a Daly at that sort of money?
 
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