JBD BMS and Battery Fully Charged

[Oceansailor]

I noticed when the Battery is fully charged the BMS turns off charging. Now the discharge is on and the current flows through the backfire Diodes of the charge circuit.
The temperature rises....at the end, the discharge switch is turned off by over temperature.
In practical terms: You leave harbour, after a while wind drops and engine is turned on.....now the alternator charges the battery. The BMS is set to 250ah, the batteries can take 300AH. The battery voltage and current are still ok. The BMS turns off because it reached the set 250ah. The BMS turns off the charge cicurit. If current is low just the voltage drops by 0.6 Volts....
Most boaters must have seen this with the common port BMS. Has anybody modified the circurit? I´m thinking of adding a relay into the discharge or add a circuit to stop operation of the DC-DC inverter (DC-DC inverter used for limiting current from alternator).
In bad weather motoring is sometime a whole day...
Wonder about you opinion?

 

[Wibla]

What kind of DC-DC charger do you have? If you set it up right, it should hit float at some preset voltage and not cook your BMS like that.

Using the BMS to control charging via FETs is really not the right way to go about it, that should be the last resort (and as you can see, it doesn't even work that well in your specific case).

 

[Oceansailor]

I know this, but I want to charge only 95 % and the BMS is capable to this by setting it to 250aH. as soon as the amount is reached the BMS turns off charging. You can over charge with 13,6 Volts...all the setup´s by voltage are rubbish. I want to charge only up to 95 %. At this point the charge current is still 40A.
Let´s take it practical. I leave harbour in the morning, battery fully charged, motor for several hours...sure I can control the charge manually, but I want it automatically by a "Battery Management System". The BMS stands not for EMERGENCY SHUT DOWN SYSTEM. I know many people think the BMS is just for safety.

 

[Wibla]

These cheap FET BMS'es are just rudimentary safety devices.

If you want to manage your charging "properly" based on shunt-measured SOC , you need a proper BMS ($$$) that communicates with the DC-DC charger to throttle the current when they hit the desired SOC.

Set your DC-DC charger to move from bulk to absorb at 14V (3.5V per cell), absorb for 15 minutes, then go to 13.2V float.

At 13.2V float your Vpc is so low (3.3V) that you are in no danger of overcharging the cells.

Lifepo4 can handle sitting at 100% SOC for a few hours just fine, they don't degrade much from it at all.

 

[Steve_S]

Terminology is killing you here.
Amp Hours is the amount of energy stored.
Amperage for Charge & Discharge are something else.
13.6V = 3.400V per cells in a 12V/4S configured battery. That is the top of the Working Voltage Range or 90% of Gross Capacity. This is a good Target Voltage.

A BMS will not limit itself by set "AH Values" it will limit itself by cell & pack voltage. the AH & Wh settings for the monitoring & display.
If you charge a battery to 13.6 Volts and it is still taking amperage, it is NOT full ! If at 13.6 Volts the Amps taken drop to 2A or lower, THEN it is saturated and full.

What can LFP output & take in.
LFP by default can output at 1C and take charge at 0.5C. For a 100AH Battery that means it can output 100A for One Hour and it can take 50A Charge for 2 hours MAXIMUM ! LFP charges from 0%SOC to 90% SOC with CC (Constant Current) then to the "DESIGNATED" 100% using VC (Constant Voltage Variable Amps) till EndAmps / Tail Current is reached which is 0.05 of the AH Rate. 100AH * 0.05 = 5A.

Designated ?
LFP Full Voltage Curve is 2.500-3.650. Working Range is 3.000-3.400 with Nominal Voltage being 3.200 Volts Per Cell (Vpc).
Users can choose where their 100% is, most folks will CAP at 3.400Vpc and designate that as 100%

 

[mikefitz]

I noticed when the Battery is fully charged the BMS turns off charging. Now the discharge is on and the current flows through the backfire Diodes of the charge circuit.
The temperature rises....at the end, the discharge switch is turned off by over temperature.

The BMS does not work this way , also any set capacity does not turn off the BMS. Are you actually seeing this type of performance or it it conjecture on your part?

As stated just set the chargers to safe voltages and everything will be OK.

Mike

 

[Oceansailor]

I can insure, you all if you set capacity to e.g. 250A and if it reaches the 250A mark it turns off charging. It surprized me as well. I did some test on a bench with a Sterling Alternator Charger 80 A and was testing the overtemp function of this charger. In addition I tried to limit the current by thiner cabling and wanted to see the affect. By this I noticed the behaviour that it discharges via the backfire diodes...
I thought charging was turned of by the following functions: Under Temp, Over voltage, Difference too big beetwen Cells and found that it also turns off when the setup amount is reached in this case 250A . I discharged and repeated it several times...the charging current was still 40A. The JBD is quite clever. I never saw it when I was on the boat.
I´m not happy with the charging scheme altogether. The Balmar solution is from my view an old solution with the external regulator. Any new car alternator uses the Smart regulation via the Linbus and Ecu does and even more than the old Balmar.....in addition the new type alternators are smaller and produce far more power, but I have not seen any kit for boats which uses the Linbus....
Balmar uses a new type alternator ....but with an external regulator for the field....that was the solution for many years ago.....technology has advanced since.
For boats, I believe it would be best to use a separate port BMS anyway, a modern small alternator fits in size (not all boat have no space for a large alternator plus serpentine belt, but the newish alternators will fit.
I like the JBD with all it´s functionality, it´s well supported on Github, but I did not look at their Competition. There are many JBD´s around.....the specialized ones I don't trust since produced only in small quantities....The large companies have their view on RV´s and not really interested in Yachting anymore.
The plan even if you disagree is to use the JBD BMS with perhaps additional hardware (Relay?) and to swap my alternator and program a ECU simulator....
I ´m certain not the first boat owner with a mid size boat (34 foot) which wants to replace the leadacid batteries, with out buying everything from one of the big companies, which have focused their mind on RV´s plus I rather buy new sails.....