Choosing Between a Single Large Lithium Battery or Two Smaller Ones

[Lenny HB]

Hydraulic, even chinese poor calibrated dyes can make a good cold weld if you practice enough,

I have a cheap Chinese one I always use a die the next down. If crimping 25 mm sq cable I use the 20mm die.

 

[Raul]

I have a cheap Chinese one I always use a die the next down. If crimping 25 mm sq cable I use the 20mm die.

You can also file the dye a bit to close further, but just tiny bit only to make it very flat one against the other.

 

[tinkertaylor]

Pull test is at least that you can do on any crips, most hydraulic crimps with dissimilar sizes you can add strands to fill the lug, and do a second crimp with a size down dye. The crimps that I don't trust are hammer type crimpers, they just deform and squeeze the lug to grab the strands. Hydraulic, even chinese poor calibrated dyes can make a good cold weld if you practice enough, and understand the tool and parts you are using. Flared lugs with long barrel are my favourite as they can accommodate 2 or more dye withs, making the crimp longer than the dye with.
Pull test its not the best test, but is at least we can do, you could measure the resistance if you have the equipment.

Best way to test would be to run the system at Full load, most likely your inverter with the maximum wattage appliance you have ( a kettle in my case) and then measure voltage drop on the low voltage side from the battery positive terminal all the way through to the inverter positive connection.

Leave you negative probe on the battery positive and then work your way through the connections on the positive circuit until you get to the inverter positive incomer, make sure the kettle still boiling while your testing.

If you have a big voltage drop then you have a lose connection, wrong cable sizing or a faulty component...

I had a 12 volt breaker that after 30 seconds of 100 amps the voltage dropped to 4 volts across the breaker (the breaker was 200A and never tripped) This cut the inverter out on low voltage.

Ps I'm in the one battery camp.

 

[Raul]

Best way to test would be to run the system at Full load, most likely your inverter with the maximum wattage appliance you have ( a kettle in my case) and then measure voltage drop on the low voltage side from the battery positive terminal all the way through to the inverter positive connection.

Leave you negative probe on the battery positive and then work your way through the connections on the positive circuit until you get to the inverter positive incomer, make sure the kettle still boiling while your testing.

If you have a big voltage drop then you have a lose connection, wrong cable sizing or a faulty component...

I had a 12 volt breaker that after 30 seconds of 100 amps the voltage dropped to 4 volts across the breaker (the breaker was 200A and never tripped) This cut the inverter out on low voltage.

Ps I'm in the one battery camp.

I like your methodology of testing for voltage drop. However, I have a resistance test meter, it shoots a pulse of 1000v AC at 100hz and measures the resistance between the points I touch the probes. That way I can measure a crimp and any crimp under 50mohms its a good crimp for low voltage. Also a voltage drop as you mentioned already, it only shows under a certain load, so you use a large or max load for a meaningful result. A resistance meter it has the ability to measure the resistance for extreme load, outside the system design capability.
Of course, the more equipment you have, the easier is to work. But, for diy most rely on a multimeter. My house runs on two batteries, ( soon to be three), two solar chargers and two inverters, all in harmony within a acceptable balance. The take from this discussion is how to wire and how to keep a balance between multiple sources. It can be done with care and high quality fittings. Weather you have one or more batteries, its another caveat, it has advantages to have multiple vs single battery, but with extra hardware implementation

 

[treblid]

However, I have a resistance test meter, it shoots a pulse of 1000v AC at 100hz

I am confused with your reply. High voltage is normally used to test insulation resistance. You need a high current (even if brief) to stress test a connection for resistance and show the voltage drop across the joint.

The problem with multimeters is that the resistance measuring test current is very low and will not show up a poor connection that would create a problem at high currents.

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[Lenny HB]

I am confused with your reply. High voltage is normally used to test insulation resistance. You need a high current (even if brief) to stress test a connection for resistance and show the voltage drop across the joint.

Didn't confuse me he was measuring the resistance, very low resistance =good joint.

 

[treblid]

Didn't confuse me he was measuring the resistance, very low resistance =good joint.

Yes, but you don't use a high voltage to do that! It can also damage the equipment under test.

 

[Raul]

Yes, but you don't use a high voltage to do that! It can also damage the equipment under test.

It doesn't damage a piece of cable with a lug,it is used to measure battery resistance, maybe not 1000v, but definitely high AC to compensate for lack of amps. Low amps high voltage or high amps low voltage to balance out.

 

[Dorwyn]

Don't know who would give advice like that. The whole point of a smart alternator is that it doesn't charge the starter battery during normal driving and cruising. It waits until you lift off the accelerator or actively apply the brakes to turn on and send charge to the battery. The idea is that it is using otherwise wasted energy by loading the engine only when it's not trying to increase or maintain speed.

In order to do this, there must be space left in the battery, so it mostly keeps the starter battery at less than full, typically 60% or so. If you connect your leisure battery in parallel with a split charge relay, it will get exactly the same treatment. Is that what you want?

A B2B takes power from the alternator in the same way that other things like lights, fans etc do. It takes power into a proper multistage smart charger which converts it into the voltage and amps that the battery needs. It's a good idea with standard alternators, and essential with smart alternators. Also a good idea with lithium batteries anyway.

Thanks for explaining that. Last trip to SW France, we did 3 days at 250 or so miles a day on motorways, using the fridge on 12 volts while driving which is what I've always done in the past, plus no mains at night on aires. The battery went while driving flat. Phoned the dealers, they said it's because of the smart alternator. That made no sense at all to me. What's smart about that? So in that case, a b2b would help?

 

[autorouter]

Last trip to SW France, we did 3 days at 250 or so miles a day on motorways, using the fridge on 12 volts while driving which is what I've always done in the past, plus no mains at night on aires. The battery went while driving flat. Phoned the dealers, they said it's because of the smart alternator. That made no sense at all to me. What's smart about that? So in that case, a b2b would help?

Yes, the smart alternator aims to keep the starter battery at about 12.6V, enough to reliably start the engine but still saving on fuel by not over-filling the battery. For a lead-acid leisure battery, 12.6V is not great but not too bad either. However a lithium battery at 12.6V is about 15% full, ie effectively flat. A B2b would give the lithium battery whatever it needed to get to 100%, no matter what the alternator voltage was.

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[Dorwyn]

Yes, the smart alternator aims to keep the starter battery at about 12.6V, enough to reliably start the engine but still saving on fuel by not over-filling the battery. For a lead-acid leisure battery, 12.6V is not great but not too bad either. However a lithium battery at 12.6V is about 15% full, ie effectively flat. A B2b would give the lithium battery whatever it needed to get to 100%, no matter what the alternator voltage was.

Mine's an AGM which I believe prefers a higher voltage than that, maybe over 14v. But should that have been fitted from new?