Thread: Battery problem?

Originally Posted by Hoges

The Victron meter is showing 13.39/13.40V both on the meter and the bluetooth app on the phone and 13.37V on two separate multimeters. This seems a significant difference from the 13.9V figure from DriveSafe.

Hi Hoges, the 13.9v I refer to is while the battery is being charged.

Your 13.39v RESTED voltage reading is fine and is normal for a good quality lithium battery in a fully charged state.

If you apply any load to that battery, it will drop to about 13.2v and stay at about that voltage until your battery is around 80% discharged.

At about 80% discharged, your battery voltage will start to drop.

Originally Posted by drivesafe

Hi Hoges, the 13.9v I refer to is while the battery is being charged.

Your 13.39v RESTED voltage reading is fine and is normal for a good quality lithium battery in a fully charged state.

If you apply any load to that battery, it will drop to about 13.2v and stay at about that voltage until your battery is around 80% discharged.

At about 80% discharged, your battery voltage will start to drop.

Ah! Thank you Tim. Ever the voice of reason and reassurance!

I don't know if this is helpfull in context, but I'll leave it here and the viewer can decide:

Hi Tin and while that guy may have had good intentions, he really has no idea what he was talking about.

He states that because his mate ( or whoever ) had used 10G cable instead of 6G cable, this would mean his battery would never fully charge

Completely wrong.

While the battery is in a low state of charge, yes it will draw the full 30 amps from the DC/DC charger and this 30 amp draw would cause the amount of voltage drop stated.

But as the battery got to around 70 to 80% charge state, the current the battery is drawing tapers off and as the current draw reduces so does the voltage drop.

So the battery will eventually be supplied will a low current charge at near the highest voltage being supplied by the DC/DC charger.

So with his limited knowledge of how batteries charge, if the battery needed that higher voltage to fully charge, it would be getting it.


Next, even at the reduced voltage, if this did not rise as the battery charged, the battery could still be fully charged with the lower voltage, it would just take a little longer.

All batteries can be fully charged with as little as 13.8v


Next, he calculated the voltage drop between the CRANKING battery and the DC/DC charger based on the cranking battery supplying the same current into the DC/DC device as that device was supplying to the AUXILIARY battery.


No DC/DC device works this way.

All DC/DC devices are inefficient and require far more current in than they can supply out to the battery being charged.

So his calculations are off by a considerable way.


If he knew this, he would have realised that what he was saying about having the charge device, be it a DC/DC charger or a solar charger, should mounted as close as possible to the battery being charged, was more important than he knew.

Originally Posted by drivesafe

Hi Tin and while that guy may have had good intentions, he really has no idea what he was talking about.

He states that because his mate ( or whoever ) had used 10G cable instead of 6G cable, this would mean his battery would never fully charge

Completely wrong.

While the battery is in a low state of charge, yes it will draw the full 30 amps from the DC/DC charger and this 30 amp draw would cause the amount of voltage drop stated.

But as the battery got to around 70 to 80% charge state, the current the battery is drawing tapers off and as the current draw reduces so does the voltage drop.

So the battery will eventually be supplied will a low current charge at near the highest voltage being supplied by the DC/DC charger.

So with his limited knowledge of how batteries charge, if the battery needed that higher voltage to fully charge, it would be getting it.


Next, even at the reduced voltage, if this did not rise as the battery charged, the battery could still be fully charged with the lower voltage, it would just take a little longer.

All batteries can be fully charged with as little as 13.8v


Next, he calculated the voltage drop between the CRANKING battery and the DC/DC charger based on the cranking battery supplying the same current into the DC/DC device as that device was supplying to the AUXILIARY battery.


No DC/DC device works this way.

All DC/DC devices are inefficient and require far more current in than they can supply out to the battery being charged.

So his calculations are off by a considerable way.


If he knew this, he would have realised that what he was saying about having the charge device, be it a DC/DC charger or a solar charger, should mounted as close as possible to the battery being charged, was more important than he knew.

Thank you for that, Tim. Maybe he should have stayed a firefighter. And maybe YOU should have a YT channel!

I don't know DC/DC systems from nuclear physics, but I always thought the current in a system is on the load side, not the charging side.

I do know that I have a lot to learn.

Originally Posted by Tins

Thank you for that, Tim. Maybe he should have stayed a firefighter. And maybe YOU should have a YT channel!

I don't know DC/DC systems from nuclear physics, but I always thought the current in a system is on the load side, not the charging side.

I do know that I have a lot to learn.

The battery being charged is the load.

Originally Posted by LuckyLes

The battery being charged is the load.

Fair enough, but I was under the impression it would only draw what it could, unlike a fridge motor, for example. You can't trickle charge a motor.

Guess i need to read up on modern systems.

Originally Posted by Tins

Thank you for that, Tim. Maybe he should have stayed a firefighter. And maybe YOU should have a YT channel!

If the manufacturers of the different products put up decent explanations of how their specific products work, it would go a long way to helping the average person both to understand how that product actually works and for people to make a far better choice when buying what ever they need.

Here are a couple of exanmples

Many people think that say if they have a 100Ah battery and they buy a 50 amp DC/DC deice and if that 100Ah battery is dead flat, the 50 amp DC/DC device will fully charge it in 2 hours.

The sellers of these devices just love that sort of misunderstanding, especially when the reality is that it will take at least 5 hours.

And the amount of times I see arguments about the required voltage needed when charging a battery.

Most read the chargers specs, where it states it chargers say an AGM battery at 14.7v and think this is the required voltage when in fact it is the maximum safe charging voltage and actually has nothing to do with a minimum charging voltage.

Another one I come across is the claim a DC/DC device with multi stage charging is a smart charging device while an alternator is a dumb charging device, yet even the old constant voltage type alternator is far superior at charging batteries that any DC/DC device on the market.

All this comes from clever advertising and not facts based info.