Dumb question here. Why do I need an SCC if there is a BMS connected?

[D308]

If the battery pack already has a BMS why do I need an SCC? Isn’t the purpose of the BMS to protect the battery from over charge and over discharge? In the past I have used a regular bench power supply to charge battery packs with a BMS. I did not have a CV CC supply. I have used a Harbor Freight SCC (Not designed for lithium) and charged a lifepo4 battery with success.
Is the SCC really needed if I am charging one 12 volt lifepo4 battery with a built in BMS?
Another question is why do we need a dc-dc converter to charge a Lifepo4 battery installed in a travel trailer from the truck electrical system?
If the vehicle charging system has an output of 13.5 to 14.5 what is the need for a dc-dc converter?

Thanks for any insight.

Don.

 

[Craig]

If the battery pack already has a BMS why do I need an SCC? Isn’t the purpose of the BMS to protect the battery from over charge and over discharge? In the past I have used a regular bench power supply to charge battery packs with a BMS. I did not have a CV CC supply. I have used a Harbor Freight SCC (Not designed for lithium) and charged a lifepo4 battery with success.
Is the SCC really needed if I am charging one 12 volt lifepo4 battery with a built in BMS?
Another question is why do we need a dc-dc converter to charge a Lifepo4 battery installed in a travel trailer from the truck electrical system?
If the vehicle charging system has an output of 13.5 to 14.5 what is the need for a dc-dc converter?

Thanks for any insight.

Don.

Not necessarily the job of the BMS is to protect each cell form overcharge/undercharge the scc should control the unit as a whole. Plus if you leave ray panel power to a bunch of batteries you could easily send too many amps to a battery at one time and wear it down faster.

 

[Dzl]

If the battery pack already has a BMS why do I need an SCC?

The simple answer is they serve different but overlapping purposes.

There are two useful ways to differentiate them.

1. Role/Responsibility/Specialization: The BMS is responsible for cell level management and battery protection, and is the only device . The charge controller is responsible for safe and efficient charging. There is overlap between these duties but their areas of responsibility and specializations are distinct and the BMS bears ultimate responsibility for battery protection and the SCC bears ultimate responsibility for charging.
2. Capability: As mentioned above, a BMS is the only component capable of cell level monitoring and protection. Other devices can see battery voltage, but not cell voltage, and thus can't protect the individual cells. Likewise, a BMS can switch off charging/discharging when voltage gets too high/low, but that's all it can do (switch charging on or off), a MPPT SCC is capable of much more sophisticated and effiicient control of solar charging.

As for a DC-DC (battery to battery) charger being necessary if you want to charge a lithium battery off your vehicles charging system (alternator/starter battery). My understanding is that the primary reason is to protect the alternator. I believe the gist of it is a lead acid battery has higher resistance than a lifepo4 battery, so a lead acid battery will limit the current, whereas a lithium will take as much as the alternator will give it and the alternator will break its back trying to supply it and possibly overheat or burnout trying.

 

[Bhupinder]

Just to top it up - SCC has array of basic functionalities to cover charging responsibility in a standalone system. In absence of any specialized control devices like BMS/ Dc-Dc converters/ Protection boards of multiple kinds, it can take-up the role of Central Control Unit for a system - especially the solar ones and keep things moving. It may not essentially be " Master of all trades" but is definitely supposed to be "Jack of all". You may encounter multiple products from vendors, offering varied set of functions 'n sophistication under the same umbrella name of "SCC".
So you get the Idea.

 

[D308]

Thanks everyone.. that helps to understand.

 

[Pierre]

Thanks everyone.. that helps to understand.

Another reason for the DC to DC is to protect the starting battery from being depleted by the load - nothing sucks more than a flat battery in a diesel vehicle.
Most DC - DC converters have a cut off voltage setting as to isolate the starting battery and most will only operate when the vehicle battery voltage is higher than the aux battery i.e. when the vehicle engine is running.
Further , the DC-DC should be mounted as close as practicable to the aux battery system as to utilize the boosted output voltage for more efficient charging as the voltage drop on your cable run from the starting battery will be compensated for.

 

[tictag]

Another reason for the DC to DC is to protect the starting battery from being depleted by the load

Whilst I do agree with this, I would just say that it is very ill advised to have a direct connection from the starter to house battery, not necessarily because the house battery might discharge the starter battery but because when you turn that key, you effectively connect an almost zero-ohm device (i.e. starter motor) to the battery(s) and you don't want your house battery trying to supply current through what will most likely be wholly unsuitable cables.

p.s. Some people argue that a VSR solution is practical (oft called 'split-charging'), and yes, that will avoid a direct connection, but, man, what if it failed? A DC-DC adaptor between starter and house systems would always be my recommended solution.

Edit: added postscript.