Please help expain this dual battery behaviour

[portjacksonday]

Hi everyone,

After not using my boat for a while, I started the engine and the starter rotated very slowly. Then I switched to battery 1. The same. Very slow. But when I switched to battery 2, it started to rotate fast and started the engine.

Two batteries: slow
Battery 1: slow
Battery 2: fast

How is it possible? Shouldn't two batteries be always better than one? These are exactly the same batteries, I bought them new a year ago. I always switch into two batteries when I use the boat.

After recharging batteries, both batteries were also slow but they start the engine lazily. I need to do something. This is not good because I don't have complete trust that I will be able to start the engine anytime when going off shore.

[NAGG]

My guess is that there is a problem with Battery 1

Normally you would only use one battery to start the boat - the other battery (often a deep cycle) is there to run electronics & supplement the start battery in an emergency .

So the way you should run dual batteries

Start - Battery 1

Motor running - Battery 1 & 2 ( both batteries are being charged)

Stopped (motor off) - Battery 2 ( running your electronics)

Restart - Battery 1


This way your batteries are both topped up but more importantly your start battery is always fresh (fully charged)

If that makes sense

Chris

[Mopheus]

B1 is dying. Could have been just low charge if it came good after charging, but it didn't so a bad battery is the logical diagnosis.

Bad B1 = slow crank
Good B2 = fast crank
B1+B2 = slow crank bcs paralleling the two results in B2 charging B1, so less energy (volts) available than when you tried starting off B2 only.

Sounds like you definitely need to replace B1 and probably B2 while you're at it. Maintain them on a smart charger when the boat isn't in use - ideally a separate charger for each battery, but in parallel (ie B1+B2) will work provided the batteries are of similar capacity.

If you're at sea with good batteries but where B1 voltage has run down and won't start the motor, switch to B2 and not B1+B2. You don't want to lose charge from the good battery into the low-voltage battery before you've started the motor.

The only time where B1+B2 could be better than starting off a good B1 or good B2 is if both batteries are run down and neither, individually, can crank the motor. Combining them both might just be enough to get you going.

[Noelm]

Possible you have a bad connection on the "slow" battery, or a dodgey cable, or the battery is faulty/not fully charged from last use, a voltage drop test will reveal possible faults, straight out testing battery voltage is pretty much a useless test.

[Dignity]

B1 (little charge) is dragging B2 (fairly good)down to it's own level - when paired, the lowest denominator wins out.
If only a year old take them back and get a heavy charge cycle put into them, it may rejuvenate them. If you're really lucky you might get some new ones under warranty, not full price but pro rata.
You didn't mention what type of batteries they were, lead acid, AGM etc and whether they are crank, deep cycle or hybrid.

A VSR would be a good investment otherwise you have to use Nagg's method and rely on memory. Depending on your motor, the larger ones often have an auxiliary charging circuit, mine does and both batteries are separated, one for cranking,one for house therefore both are being charged without me having to think about it, also keeps curcuits very clean and separate from each other.

[tunaticer]

Check your ground wire for starters, then check the connections at the switch.
Then disconnect both batteries at the battery terminals and load test them individually....do not rely on the battery isolation switch to switch batteries for testing....they share a common ground.

Most likely it is a ground problem, possibly in the battery isolation switch.

[Dignity]

Check your ground wire for starters, then check the connections at the switch.
Then disconnect both batteries at the battery terminals and load test them individually....do not rely on the battery isolation switch to switch batteries for testing....they share a common ground.

Most likely it is a ground problem, possibly in the battery isolation switch.

I forgot about this first step. I had what looked like good solid connections a year or so ago but the wire had separated at the lug under the heatshrink. I'd used cable from a welder, I suspect the individual wires were too fine, carried current great when installed but degraded over time.

Follow Tunaticers advice.

[gazza2006au]

Hi everyone,

After not using my boat for a while, I started the engine and the starter rotated very slowly. Then I switched to battery 1. The same. Very slow. But when I switched to battery 2, it started to rotate fast and started the engine.



Charge both batterie's let them sit for 48 hours and check there voltage, Sound's like battery 1 may need a recharge or if it's reading 10.8volt's after sitting it's dropped a cell

[stevej]

Standing voltage is half the story
need to se what they do with a load applied

[ericcs]

if they are only a year old, they should still be under warranty. maybe you can take them back where you bought them and see if they can charge and load test them?

[portjacksonday]

Thanks, guys. You gave me plenty info to work on and check. I know that both batteries are Hybrid. I will need to check the other details.

[Dignity]

Thanks, guys. You gave me plenty info to work on and check. I know that both batteries are Hybrid. I will need to check the other details.

If they're hybrid, a word of warning, they contain calcium and batteries that contain calcium do not like being discharged below DOD at all so the chances are batt ! fell below DOD while in storage and will in all likelihood not recover. One of the reasons I avoid them.

[Spaniard_King]

Easy to diagnose with a good multi meter and a bit of 12 V knowledge

only testing under load will you find the fault (cranking engine)

using the good battery as the Home base (for ease of terminology)

Negative on good battery and positive on good battery positive (now crank and record reading) This should be highest reading

Negative on suspect battery and positive on suspect battery positive (now crank and record reading) This should be lowest reading

Negative on good battery and positive on suspect battery positive (now crank and record reading) This confirms how good the positive circuit is on the suspect battery

Positive on good battery and Negative on suspect battery negative (now crank and record reading) This confirms how good the Negative circuit is on the suspect battery

This should give you an indication of what your circuits are like

[portjacksonday]

I haven't done everything yet. I am still in progress. A bit busy at the moment with other things in life. However, I found one definite problem. 1/3 of wires in battery 2 lug connector were broken. I suspect the cable was bent in opposite directions because the battery box was slightly bigger than the battery. When I cranked the engine a few times, the lug became hot.

I cut the cable and used a propane burner to melt the solder and I attached the lug to the fresh wires. I see better results already but I will need to charge the battery before I can conclude that it was the only problem.

A friend of mine told me that what I did was reckless because I didn't think about the other end of the power cable and the heat could burn something there. But so far so good. I don't see any problems yet. Do you think this is a valid concern for the next time if I need to do it again?

[portjacksonday]

I have fixed the problem finally. The batteries were good. I blame the battery cables (gauge-2) going from the batteries to the switch. I think they got oxidized over time. Replacing the cables fixed it. I also replaced the switch before but it didn't help. The voltage drop method helped find the problem. It turns out those cables have expiry date especially when exposed to the elements. Thanks again, everyone.

[Noelm]

Yep, voltage drop test us the only way to test, standing voltage test is useless, charging batteries and rooting around just makes things more confusing, faults like you describe are nearly always a cable or connector/connection issue