Chargers - are they necessary

[thedodgersonline]

Hi All.
We all have read the discussions - often heated - about the necessity of a BMS in a Li system but what about the necessity of a charger (solar MPPT or dc to dc) when there is a BMS already handling the CC/CV of the battery. Now I am not talking about a ziggawat solar system with pentawh of storage but a simple 12V LiFePO4 system used in a RV of small home. I am sure I am missing something (and not about to wreck my brand new LiFePO4 battery pack) but I cannot see any downside to this approach. An alternator produces approx 15V and some amps to the BMS which then charges the battery according to its parameters! Same with a solar panel(s).

Can the brains trust PLEASE enlighten me

Thanks and Regards

 

[MisterSandals]

what about the necessity of a charger (solar MPPT or dc to dc)

If you are at all interested in specifying the charge current and voltage, I'd say a charger is necessary.
These things interest me mostly because i do not want to ruin my expensive batteries. And so I can count on them being charged.

Otherwise chargers are not much use at all.

 

[MisterSandals]

An alternator produces approx 15V and some amps to the BMS which then charges the battery

Regarding alternators, most are not designed to be connected to a batteries that accept current as readily as LiFePO4. Alternators will work harder than they are intended and if not cooled, they will burn out. DC to DC chargers limit the current flow from the alternator so it does not get overworked. And like other chargers, they control the current and voltage to properly charge the receiving battery.

 

[MisterSandals]

the BMS which then charges the battery according to its parameters!

Not true! The BMS is a switch. If the out of bounds conditions are exceeded, it opens the contacts. It does not know or care what the current going thru it does ... it can be on the charge side (between charger and battery) or on the load side (between battery and load).

The BMS is a switch.

 

[thedodgersonline]

Regarding alternators, most are not designed to be connected to a batteries that accept current as readily as LiFePO4. Alternators will work harder than they are intended and if not cooled, they will burn out. DC to DC chargers limit the current flow from the alternator so it does not get overworked. And like other chargers, they control the current and voltage to properly charge the receiving battery.

The charge current is specified in the BMS so it will not overload the alternator.

 

[thedodgersonline]

Not true! The BMS is a switch. If the out of bounds conditions are exceeded, it opens the contacts. It does not know or care what the current going thru it does ... it can be on the charge side (between charger and battery) or on the load side (between battery and load).

The BMS is a switch.

You can set up all the charge / discharge / protection parameters in the BMS and when the charge / discharge reaches those limits the BMS stops. If you set the charging current to 10amps and connect a 20amp charger the BMS will limit the current to the 10amps, similiar I’ll with the discharge current. You can have a 120a mp rated BMS and limit the max discharge to 30 amps.
Or am I wrong in all this?

 

[upnorthandpersonal]

An alternator produces approx 15V

The maximum you can charge a 4 cell LiFePO4 to is 14.5V, 14.6V if you really need the last bits of power (https://diysolarforum.com/ewr-carta/diy-lifepo4-charge-profile-parameters/), meaning putting in 15V is too much and will damage your battery.

Edit: also, one should not consider a BMS to be a device that does switching and regulating at normal operating parameters on day to day operation. A BMS is intended to be a protection device that only kicks in when things go wrong.

 

[Dzl]

Hi All.
We all have read the discussions - often heated - about the necessity of a BMS in a Li system but what about the necessity of a charger (solar MPPT or dc to dc) when there is a BMS already handling the CC/CV of the battery.

What BMS are you using?

 

[thedodgersonline]

What BMS are you using?

The DGJBD reviewed in this thread - https://diysolarforum.com/threads/dgjbd-bluetooth-bms-a-short-review.5290/

 

[MisterSandals]

If you set the charging current to 10amps and connect a 20amp charger the BMS will limit the current to the 10amps, similiar I’ll with the discharge current. You can have a 120a mp rated BMS and limit the max discharge to 30 amps.
Or am I wrong in all this?

Hmmm, this would be news to me. My understanding was that if the charge current was too high it would open the circuit rather than do actual current limiting.
Maybe this is the thing i learn today?

 

[upnorthandpersonal]

It won't limit the current though: if a higher current appears, it disconnects the battery to protect it. You can configure the trigger value.

 

[Dzl]

Hmmm, this would be news to me. My understanding was that if the charge current was too high it would open the circuit rather than do actual current limiting.
Maybe this is the thing i learn today?

This is my understanding as well.

 

[Sabre36]

The charge current is specified in the BMS so it will not overload the alternator.

Please udnerstand that there are two types of internal FET based BMS's.

#1 Catastrophic protection only: It is not designed to really care for the cells just to prevent them from thermal runaway. These are the BMS systems that have extremely high HVC eg; 15.5V - 16.0V and extremely low LVC eg; 10V or lower.

#2 Safer for the cells BMS: These BMS's attempt to protect the cells for maximum longevity and have significantly lower HVC and significantly higher LVC.

There are also multiple specs for most internal FET based BMS systems. The recommended charge current eg; 0.3C and the max brief charge current eg; 1C. There is also the max safe voltage, max cell high temp and sometimes a cell low temp cut off. Some also have maximum current limits which can cause the BMS to open to prevent damaging the BMS. Some also include a cut off based on the BMS temp, if you drive it too hard. This spec is often mysterious, and not even specified in most spec sheets, but if you load one up and watch the BMS temp they can open even if the BMS is below the max current limit, for long enough.

If the BMS opens while your alternator is charging, for cell temp, max current limit exceeded, BMS temp or high voltage, you can blow the alternator diodes due to the load (battery) being disconnected. This voltage transient also winds up on the DC bus and can damage sensitive equipment too.

The safe way to manage an alternator is to open the + power feed to the regulator, using a signal from the BMS, which cuts regulator power slightly before the BMS causes a load dump. Unfortunately there are very few BMS systems that can do this as most of them do one "cut" and have no way to shut down the alternator slightly before the battery is disconnected..

With drop-in batteries the internal BMS does not "manage" charging, it is the job of the charge source to do this and ensure that BMS parameters are not exceeded.

An internal BMS simply disconnects when out of range parameters are exceeded. A higher quality BMS, such as a REC-BMS, can talk to the charge sources via CAN and directly control an external regulator, such as a Wakespeed WS500. The WS500 then protects the alternator from heat damage.

There are ways to mitigate a load dump but the internal BMS should not be used to regulate charging.

 

[John Frum]

The charge current is specified in the BMS so it will not overload the alternator.

What BMS does that?

 

[mapguy525]

You can set up all the charge / discharge / protection parameters in the BMS and when the charge / discharge reaches those limits the BMS stops. If you set the charging current to 10amps and connect a 20amp charger the BMS will limit the current to the 10amps, similiar I’ll with the discharge current. You can have a 120a mp rated BMS and limit the max discharge to 30 amps.
Or am I wrong in all this?

I think you need to do some additional research on how the different layers of an electrical system interact on a typical LFP installation with multiple charging sources. Each charging source is an individual layer of your system that needs to be harmonized with your exact system variables.

This link can help find the information you need to make sure your batteries and electronic devices survive. I would review the article at Marine How to website
https://diysolarforum.com/threads/alternator-charging.6465/

 

[thedodgersonline]

Hi Everyone.
Thanks so much for the input. The issue with newbies like me is moving to conscious incompetence from unconscious incompetence. I definitely did not over think this for sure.
Will do lots more reading and watching.
PS I do have a MPPT and DCDC into my battery - was just a thread I could not substantiate.
Regards

 

[Dzl]

The issue with newbies like me is moving to conscious incompetence from unconscious incompetence.

haha, i feel your pain here, the more you learn, the more you understand just how little you know and how things you thought you knew are actually more complicated than they first seemed