Inverter and 1 wet battery

[ToBoldlyGo]

I have 1 flooded 75ah battery. When not on ehu the main electric consumers I want to cater for are our non-led lights and our 320/1000watt (heat/electrical power) microwave.

The battery is in a sealed sunken box, with a cable coming out of a hole in the box lid and a fuse on that cabke outside of the box. The box size limits me to about one 75-80ah battery.

My electrical knowledge is O level and very basic. There are 2 routes for me to take that I can see, 1 to replace my existing single battery with 2-3 agm, gel or lithium or 2 keep my 1 battery (microwave will only be used a few mins a day and I can upgrade to led lights).

In the short term I would quite like just to connect my inverter to my battery. Initially I thought I could just open the box lid and connector the inverter to the battery using the leads that came with inverter. However I now think the simplest I can do is:

1 drill a second hole in the battery box
2 run extra high power leads from battery through the new hole and attach them to fuse
3 attach fuse to inverter and the inverter will be mounted outside the box, either inside the lounge underseat compartment or screwed to the side of it

My question is - is this the least I need to do, to keep inverter away from hydrogen?, because if it is it seems a lot to do to keep my inadequate battery set up.

I have just thought of a variation on steps 1-3, ie do not do steps 1 and 2 but instead get an electrician to attach a cable to the exsting fuse outside the battery box and the use that to separately fuse the inverter.

 

[funflair]

In my opinion your single battery is no where near big enough for the inverter so I would go straight to Plan B.

 

[Itsmcb]

As above. A fully single 75 amp battery actually has only around 35amps of usable power. A standard 700 watt microwave draws about 120 amps from the battery so your battery would be completely drained in 15 mins.

 

[Lenny HB]

The max rate you should discharge a lead acid battery at is the C5 rate for your battery that is 15 amps. Running your microwave will draw around 120 amps.
AGM's are a waste of time as a leisure battery, to run your microwave you will need 200ah of lithium.

 

[autorouter]

My question is - is this the least I need to do, to keep inverter away from hydrogen?, because if it is it seems a lot to do to keep my inadequate battery set up.

When you say it's a flooded battery. is it the type with six vents on the top, that allows topping up with water? This type of battery produces hydrogen, as you say, as part of its normal charge/discharge cycle. There are other types that don't, and are sealed, with a pressure release valve in case of a battery fault. Some flooded types, and also AGM and Gel, are the sealed type.

Sealed batteries are allowed in the habitation area without a sealed box, although most people fit a thin plastic tube to the pressure release vent just in case. The tube vents through a small hole in the floor.

If you want to use a microwave, that takes a heavy current for a few minutes. A 700W microwave outputs 700W cooking power, but it needs more power on the input, more like 1000W. From your O level physics (watts = volts x amps) you can work out that 1000W from a 12V battery requires 1000/12 = 83 amps.

There's two considerations with a battery. If it's a 100Ah capacity, then 83 amps for 6 minutes (1/10 of an hour) is going to use 83/10 = 8.3Ah. That's OK for a 100Ah battery, it won't drain the capacity too much. But the other consideration is the maximum amps you can draw from a battery at any one time. As Lenny HB says, that should be limited to C/5 where C is the capacity. So for a 100Ah battery the limit is 100/5 = 20A. So your 83A for the microwave is definitely over the top.

If you definitely want a high-power device like a microwave, you can add extra batteries. a 400Ah battery bank would increase your limit to 400/5 = 80A, and would be fine. Some people find that 300Ah or even 200Ah of battery isn't too bad either. It's not the best for optimum battery life, but they can live with it.

Another option is to switch to a lithium battery, and most of them have a much higher charge/discharge limit. Many 100Ah lithium batteries can be charged and discharged at up to 100A without affecting their life significantly. But they are expensive, and you may need to get different charging devices (mains, solar, B2B) if the existing ones don't have lithium settings. The other benefit is that lithium batteries are smaller and lighter for the same capacity, so quite likely you can fit a bigger capacity lithium battery into the existing battery space.

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