High level electrical design question - inverter power

[roland stockham]

Hi all, I know there are some good electrical experts on her and this is a question for you.

I have been running a 1500w (cont) power 24ov inverter that has worked fine and does most of what I need but it has dies and needs replacing. What I wont to do is up the power to 3-4Kw so that I can run a min welder for small jobs. Need to be able to run 7A for up to 15min but likely in short bursts of no more than 2-3 min. That's the goal
The problem is that the 12v electrical system system can only really work up to about 150A and the new inverter will draw 250-300A which would simply crash the voltage from the current battery. I could change the battery set up to Lithium but to do that properly would be thousands and not worth it just to occasionally run the welder. The alternative would be to double the size of the current house bank but again that's well over the $1000 mark and it would be a challenge to find room for that much capacity. Also 90% of the time it would be unnecessary and overkill.

What I am thinking about is can I use a start battery with a CCA rating of around 900a to boost the short term current available. I want to keep the inverter running of the house bank most of the time but to just add a boost setting when I need very high power.
The solution I am considering is to add a second start batter to the current start circuit so that it would charge with the existing start battery then have a switch that would gang it to the house bank when I needed the boost. This would mean that the inverter is then going to draw most of its current from the starter batteries which would easily supply enough for a short period. There are no sensitive electronics on the house bank as I have a separate battery for the nav system, radios and computer to protect it from spikes caused by machinery so that is not a problem. Obviously I could only use this boost for specific short term jobs, if left switched in the start batt would be cycling with the house bank and soon die so would have to remember to switch it out again! small price to pay considering the cost savings.
What does anyone think, is this just unconventional or have I missed something that would mean it would not work!

[smac999]

How big is the existing house bank?

Could you just run the engine when welding? With the alt running the battery draw will be much less.

Ie 80a alt + 250a inverter draw= 170a battery draw.

[roland stockham]

Thought of that. House bank is 375A/hr plus a 175A/hr electronics battery. With the engine running these are connected via the VSR relays as is the start battery. So in theory this gives me enough power. However there are 2 drawbacks. One is that if I am on the hard or against a wall to do repairs I can't run the engine, no cooling. Two is that I could very easily fry the VSR as they are only 100A. Both start battery and the electronics battery could provide more than this if the house bank is getting low.

[EngineerRetired]

First, what is it you are welding? What sort of welder? TIG, MIG, Stick, Wire feed? If you are on the hard the simplest and most cost effective answer is to rent a small portable gen for a day or two.

[Emmalina]

+1 on the portable genset Thats what I use. Although I have welded using my inverter (2kw)

[CruiseN]

Originally Posted by Emmalina

+1 on the portable genset Thats what I use. Although I have welded using my inverter (2kw)

+1 again.

Just reminded in another thread about water + electricity.
Took a step back. Looked. I don't need mains. Led tv is 12 vDC. Blender can be rearranged to DC.
A generator ain't electrical is stationary getting bounced around in a fun zone. But without means to electrify inside hull accidentally during passages of urgency.
12,24 vDC shouldn't or won't hurt. Small inverter can be switched off but better than a large supply.
Generator weighs more safer in electrical aspects of removing hazards.

[roland stockham]

Originally Posted by EngineerRetired

First, what is it you are welding? What sort of welder? TIG, MIG, Stick, Wire feed? If you are on the hard the simplest and most cost effective answer is to rent a small portable gen for a day or two.

Agree that for anything major using a genset or yard supply is the way to go (I have a 5kw set that powers the workshop). Looking to be able to do small 'get you home' repairs when away from base such as repairing a bracket. Max 1/4 plate mild steel or 1/8 stainless using stick. My cruising ground is Canadian NW and will be based at a marina but if I head south the next marina or yard is about 900 miles on Vancouver island, heading north it is probably Anchorage which is even further. Sailing this coast is 'frontier country' you have to be totally self sufficient. Also the boat is currently stuck in Panama waiting for COVID restrictions to end. Expecting to have a fair bit to do to get her ready to leave and the storage yard only has 120v power available via a couple of long extension leads. Not even sure I can hire a geny there and the little portable one on the boat is only good for battery charging.

[wingssail]

My approach, though not recommended by anyone else on CF, is a 1-2-Both battery switch and the inverter runs off the house supply. So when, and if, I want more amps to run a welder, for example, I simply switch to "both". There are no VSR or isolators involved.

But you have to take care not to leave that switch on "both" which might run down all the batteries on board, nor have I switched it to off when the engine is running which might blow alternator diodes. In 35 years that has not happened.

[roland stockham]

Thanks wingssail that's,s a really good suggestion and I had not thought of doing it that way. Has the added bonus that when I leave the boat I can simply switch it to both and the solar panels will be keeping all the batteries topped up.

The one issue I am still wondering about is how the start batteries will do. The CCA rating gives effectively the max current the batt will supply but it is for 30 sec and allows for a volt drop to 8.5v. If I have a start batt with a CCA of 900a will it supply 150a for 5min with a voltage above 11.5? anyone tried drawing this sort of current from a start batt and if so what did you see?

[StuM]

Originally Posted by roland stockham

If I have a start batt with a CCA of 900a will it supply 150a for 5min with a voltage above 11.5?

never treid it, but I doubt it.
That's about the same draw as the starter on a small 4 cylinder car.
Of course, it depends on how many AH as well, but try cranking a car for five minutes and see what it does to the battery

[EngineerRetired]

So you are away from any real power and MUST do a little welding. Connect two 12v batteries in series (24v) and weld (stick) with a pair of jumper cables. Yes I am serious. Make SURE you are using the positive lead as ground, you must do reverse (negative) DC welding or you get garbage.
In fact I think there is a demo on YouTube. Yup, watch this...

https://www.youtube.com/watch?v=PV5oLPLUzrM&t=340s

[s/v Jedi]

Use a Victron inverter/charger with it’s Power Assist feature in combination with a small external power source (portable generator) that can handle the avg consumption, but not the peaks like when actually welding.

The invert/charger will be charging the batteries as well as powering the welder. When you actually weld, all generator power goes to the welder plus the inverter/charger adds whatever is needed extra from the batteries.

Example: welder needs 3000W, your generator can only supply 2000W: when the welder is idling, 2000W goes into charging the batteries; with welder active, 2000W goes to welder plus 1000W is added by the inverter, using batteries to get that power from.