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evm1024 · 29-04-2021, 09:36

Quite right Don. I think that this discussion is a theoretical one on what limits the charge current and targeted my comments to that end. In my upcoming charge test I'll up the inverter/charge current limit to the chargers max.

On the practical side as you noted, limiting the charge current will enhance the life of your alternator, etc.

During the charge cycle I expect to do at "full" current I'll use an IR temp probe to check the temp of the connectors to verify that they are still low resistance connections.

Even If one is running at less than 100% charge capacity is does make sense to keep the connections an low resistance as possible to maximize the charge to the batteries (and less waste heat in the cables).

Ulstue · 29-04-2021, 13:31

evm1024, your test results will be very welcome!

I would find it interesting to see how the charge current behaves with a CV 14 or 14.2V, because then one could learn how big an alternator would have to be, to charge without risk of overload.
If I understand correctly, the boost phase on Victron is the CC phase and the absorption phase is constant voltage. I think it is also possible to turn off the boost phase on Victron controllers and use only the absorption phase for charging. Unfortunately I don't have such a great Victron charger.

Oceanride007 · 29-04-2021, 15:00

I get 30 Amps out of my 55 Amp alternator, when charging my 480Ahrs LFP. I monitor case temperature and never (Intention) to do it while idling, not for long anyway, This is with a paralleling switch, and try to remember to open my switch when Engine turned off. This mode of operating would be exceptional period of no Solar.

GoneDiving · 29-04-2021, 15:23

If you are asking the ability of cells to accept current, it's huge. As mentioned above Current = Voltage/Resistance so for a 1.2v difference and a 0.4 mOhm pack, that's 3000A. That's not a typo and would likely exceed any connected wiring etc as well as the ratings of the battery for extended life. But it would be an exciting first charge!!

If you are asking about the ability of the alternator to provide charge, then you have already said that yours can deliver 80A. As this is well below the battery's ability to absorb charge, this is what you will get. At least until the alt overheats.

evm1024 · 29-04-2021, 15:30

I think that there is some confusion as to what CC and CV is. Perhaps we will address that when I finish up charging my house bank and have the graphs to show the voltage and current at various SOC.

seabreez · 29-04-2021, 15:30

[QUOTE=donradcliffe; (edited) In my boat I never saw more than 70 amps out of the 100 amp alternator with a 400 ahr gel bank, and my alternator never failed in 15 years and 3000 engine hours. For a while I tried to look at voltage drops and get more out of the alternator, but I finally said, "why?"[/QUOTE]

You understand very well... your system never needed 100 amps, that is why it only delivered 70 amps. ALL alternators will put out the amperage that they are designed to deliver. I know!!! I rebuild them and test them for every one that really want to see with their eyes!! When boaters test them they want to know what is not important, they want to know the amps, the voltage, the time, etc... Every boater has his own system and a lot of time the systems are not properly designed. The IMPORTANT thing is to have a system that will charge your batteries with every thing in the "ON" position and still have a voltage sufficient to supply your load and still be strong enough to fill your batteries to the max. So this is how you test your system... with a full charged system of batteries, light up every thing on your boat that takes power from the batteries and observe the voltage at your batteries. The voltage should be at 13,2 volts to 14.7 volts. This is a simple to test. If the voltage is 13.2 to 14,7 volts, you alternator is supplying your electrical load and the surplus is going to the batteries. If the voltage drops below 13.2, the alternator cannot supply your electrical load and therefore the battery voltage will drop. If the voltage stays up. you have a good system as not all your electrical load will be on at any given time when boating. Keep it simple. If this is not clear for you... email me for clarification... Before you test, check the tightness of your belts. Make sure they are very tight. I sometimes test with the customer and then tighten belts and retest. Very often the belts were loose for a while.

GoneDiving · 29-04-2021, 15:38

Quote:

Originally Posted by donradcliffe

If the battery is at 50% SOC, I measure 65 amps into the battery from the alternator. I have voltage drops through the relay and probably 25 feet of wire round trip. Guess what? I am very happy with the 65 amps, because it will fully charge my battery in 2 hours of driving, gets nowhere close to the 120 amp limiit of my BMS, and I am not concerned at all with burning out my expensive alternator.

What do you get as a final pack voltage? Is it limited by your "dumb" regulator minus any losses or your BMS?

I like the simplicity. 1/3C charging and 3.4 to 3.5 v/cell is good for longevity

Thanks

Ulstue · 29-04-2021, 23:31

The super small resistance given in the specifications of LFP cells (EVE, Lishen, CATL) is an AC impedanz at 1Khz (at 1C? can't remember) of an cell with 3.3V.

I don't know what that says about the internal resistance when charging with DC 14V.

I have read the internal resistance (LFP) decreases as the current increases. And to charge with 1C a voltage of 16V is needed. That seems to me to need a higher voltage for this than the alternator usually can put out.

Sorry for not being able to express better, I hope it is understandable enough.

evm1024 · 30-04-2021, 01:28

Quote:

Originally Posted by Ulstue

The super small resistance given in the specifications of LFP cells (EVE, Lishen, CATL) is an AC impedanz at 1Khz (at 1C? can't remember) of an cell with 3.3V.

I don't know what that says about the internal resistance when charging with DC 14V.

I have read the internal resistance (LFP) decreases as the current increases. And to charge with 1C a voltage of 16V is needed. That seems to me to need a higher voltage for this than the alternator usually can put out.

Sorry for not being able to express better, I hope it is understandable enough.

Current is dependent. If you want to change the current you either change the voltage or you change the resistance. Hard to change the resistance of a cell so that leaved you voltage.

It sounds simple to change the voltage right? You just set the voltage and your supply puts out that voltage. But it does not work that way. The supply (alternator, charger etc) all have limits of the current they can supply and when you reach the current limit the voltage has to drop in accordance with ohms law.

evm1024 · 30-04-2021, 01:47

Here is a photo of the charge cycle. On the right you can see that the Victron is inverting and the voltage of the house bank is dropping. I'm just running the fridge and a few AC loads so the current draw is not much.

I've set the charger voltage to 14.00 volts and the max current to 120 amps.

As I turn on the charger it goes into bulk mode and starts charging at a constant 110 Amps. This is Constant Current mode. Notice that the voltage is rising but is not at 14 volts. It is attempting to reach 14 volts but it just can't the load (resistance) is greater that the chargers ability to supply. If I had a larger charger the same would happen - more current would flow, the voltage rise quicker to 14 volts but it would not start at 14 volts.

As the battery charges (increasing SOC to near 100%) the voltage rises and reaches 14.00 volts. The charger now is in Constant Voltage mode and is actively regulating voltage. You can see the current decrease with increasing battery SOC. That portion is indicated and the absorption mode in the charger.

Lastly, based on the parameters I set in the charger the time in CV (absorption) is limited to about 30 minutes. By then the charge current has dropped to 50 amps or so and the charger enters float mode at about 13.3 volts and near zero current.