Battery dead???

[roaringgirl]

If that is what truly happens then several things are likely to be broken.
Getting 20A out of a 20A alternator for the whole of a charge cycle is vitually (I'd say definitely but someone will prove me wrong) impossible.
If the shunt is reading 20A for the whole cycle than the shunt or meter is kaput.

My batteries are LFP, and what I've described is completely normal and expected behaviour.

 

[Sandy]

LFP is lithium iron phosphate and outside my area.

 

[roaringgirl]

LFP is lithium iron phosphate and outside my area.

Voltage during charge for LFP cell

 

[William_H]

The voltage reading at a charging source will be the output voltage of the charging source. The SOC is irrelevant, save for any influence that may have to the controller/regulator of the charging source. The power of the alternator has no bearing, power out is amps, not volts, a 25a alternator can output exactly the same volts as it's 125a equivalent.

I beg to disagree. An alternator provides voltage and current as a function of alternator speed, the power of the field coil and of course the size of the alternator. The regulator senses the output voltage and reduces the field current to reduce alternator output voltage. That is the voltage being produced at the output (hence battery terminals) at the current flowing. So a small alternator connected to large flat batteries will typically be pushing out all the current it can with field coil current max. (especially at low RPM) The current flowing will dictate the voltage it can produce. Obviously higher current lower voltage so lower current tends to balance itself at best capability. Eventually voltage of battery will rise, current in will fall and alternator can then provide the voltage that the regulator is set at (14v)
A solar charging source will be even more limited by it's capabilities. A battery charger from mains power again may or may not in practical terms be limited in power capability.
ol'will

 

[Refueler]

All comes down to what are termed :

CCC and CVC

Constant Current Charge
Constant Voltage Charge

Most common chargers in use regardless of battery format are CCC for most of the charge cycle from low to near charged state, then as Current falls off - CVC takes over.

You can see this on the ammeter display of your car battery charger. When battery is low ... the needle is way up on scale and stays there until battery is approaching high charge state ... then you see the needle dropping back until it drops to its maintenance / float rate or cuts out.

 

[Refueler]

Voltage during charge for LFP cell

View attachment 143276

The charging regime for Lithium based is different to standard Floaded Acid Batterys. Due to the Voltage curve of Lithium. If you plotted a Floaded Acid battery - you would see the red line (battery voltage) have a seriously steep incline instead of the Lithium shallow incline.

 

[roaringgirl]

The charging regime for Lithium based is different to standard Floaded Acid Batterys. Due to the Voltage curve of Lithium. If you plotted a Floaded Acid battery - you would see the red line (battery voltage) have a seriously steep incline instead of the Lithium shallow incline.

Absolutely you would - the point being the alternator output voltage isn't always 14.

 

[Refueler]

Absolutely you would - the point being the alternator output voltage isn't always 14.

Do not confuse Alternator with Battery.

 

[roaringgirl]

Do not confuse Alternator with Battery.

They're in a circuit together - how can you separate them?

 

[Refueler]

They're in a circuit together - how can you separate them?

You cannot - but what you are ignoring is that one is in 'one direction' while other is opposing.

Think of it like this :

Alternator is trying to push 14V left to right.
But battery is opposing and pushing its lower voltage right to left...
Sound daft ? But look at your connectors ... its not the simple + to - circuit ... its + to + ... - to - .....
So when you put your meter probes into the circuit - what will you read ??

 

[PaulRainbow]

Hmm, until my batteries are at 99-100% full, with my 20A alternator running, the 'output' voltage is 13.2V, it creeps up as the SOC increases. At 95% it is 13.5V the current flowing into the battery as measured by the shunt is a steady 20A the whole time. Once the battery is full the voltage goes up to 14.2.

Does this mean my alternator is broken?

No, it means you failed to mention you have Lithium batteries, in a thread about lead acid batteries.

 

[PaulRainbow]

Absolutely you would - the point being the alternator output voltage isn't always 14.

No one said it was.

 

[Refueler]

No one said it was.

Its understandable actually .. I badly worded my post earlier ....

???? Alternator should pump out over 14V whatever state battery is in.

He ignored my use of the word "SHOULD" .... and I 'should' have been more careful ....