Second Alternator Charging Lithium Battery Bank

[DiploStrat]

Most lithium iron batteries can be charged with the output of most modern alternators. (Note the caveat, "most.") A decent BMS will act as a master fuse to protect against over voltage, over cell voltage, over discharge, temperature, etc. The two caveats that I would offer, one from experience:

-- A discharged lithium battery is like a back hole for amps. (That is why they charge so fast.) But a 100Ah battery, discharged down to the BMS disconnect point, can easily pull over 80A and may pull up to its full rating. This can melt your wires and fry your alternator. Most "normal" alternators should not be loaded to more than about 50% full time.

-- If something goes wrong and your BMS shuts down, it may generate a spike that may destroy your alternator. AB2B will prevent this. Having a lead acid battery in the circuit, e.g., your starter battery may also work.

Finally, there is a lot of chatter in the lithium world to the effect that lithium batteries actually like to be charged a bit slower.

As always, free advice is worth ...

 

[RAM5500 CAMPERTHING]

Sterling B2B units are available in more sizes and are infinitely adjustable, should you wish. (And know what you are doing.) They tend to be physically larger and require lots of air flow.

Good point

The Sterlings are fairly large physically and would be a nightmare to try to mount under a hood. Lucky, I have ample room Inside my box

 

[Rando]

Before investing in a B2B, the best option would be to try your charging system with your lithium battery. If your alternator output voltage is high, and you have short and heavy wiring, then there is a slight chance of drawing more current than you want - but a test will reveal this. Assuming you have loads on the battery, a voltage spike is unlikely. However, if you are worried about this, then a transient voltage suppressor is the way to address this, not a B2B. The usual issue is not enough current to the battery - which a B2B can address.

 

[DiploStrat]

Would not ’running loads’ of a vehicle what includes starter battery be sufficient to prevent alternator voltage spike if the lithium BMS suddenly disconnects ?

I would think so, and I read to that effect, but, given the chance of destroying an alternator, I don't want to recommend that anyone try it.

 

[Rando]

It is also not entirely clear how a B2B charger would protect against a BMS shutdown (aka a 'load dump')?

The issue here is not so much that you will damage the alternator, but that when you suddenly disconnect a large load from the alternator, the output voltage of the alternator will spike due to the inductance in the field windings. It is primarily other things attached to the same circuit (eg ECUs, accessories, maybe the B2B itself) that would be damaged. If the battery were to suddenly disconnect from the B2B, the load at the input to the B2B will also suddenly drop - leading to the same situation. Littlefuse has a couple of good application note on this, and how to protect against it.

 

[DiploStrat]

FWIW - I have had the BMS shut down and there has been no impact on either the REDARC B2B or the Victron solar controller. REDARC, and, I believe Sterling Power as well, specifically state that their B2B protects against load dumps.

Sterling Power sells an Alternator Protection Device to address this issue when not using a B2B: Alternator Protection Device – Sterling Power Products (sterling-power.com)

 

[DaveInDenver]

It is also not entirely clear how a B2B charger would protect against a BMS shutdown (aka a 'load dump')?

The issue here is not so much that you will damage the alternator, but that when you suddenly disconnect a large load from the alternator, the output voltage of the alternator will spike due to the inductance in the field windings. It is primarily other things attached to the same circuit (eg ECUs, accessories, maybe the B2B itself) that would be damaged. If the battery were to suddenly disconnect from the B2B, the load at the input to the B2B will also suddenly drop - leading to the same situation. Littlefuse has a couple of good application note on this, and how to protect against it.

Does seem like either a BMS or DC-DC (B2B) power supply could potentially cause or prevent the alternator from going out of regulation. It's not the load dump per say but the sudden change in feedback (sense) to the alternator voltage regulator. It's the same damage that could occur if you were to pull the battery cable off while the engine was running, which isn't an unknown failure mode.

Automotive load dump TVS – A unique transient voltage suppression application - Power Electronic Tips

www.powerelectronictips.com

It would completely depend on the BMS or DC-DC as to whether they still present a sufficient load to the source, such as input filtering or OV protection like the TVS you mention, for this not to occur. If the designer assumed it would always have a stable supply just parallel to the intact stock charging system they certainly may not do anything to prevent going off the rails. In that case if one wanted to dedicate an alternator to a BMS you could perhaps use a TVS or active load to stay in regulation or simply hang a small battery on the alternator side of the BMS or B2B?

 

[DiploStrat]

Worth noting in this:

-- REDARC, and, I suspect, Sterling Power, require that their B2B be fed from a battery, typically a lead acid starter battery. So the loss of the second (camper/house) battery would be on the other side of the B2B.
-- Neither will provide any protection to the alternator from a problem with the starter battery, which is directly connected to the alternator.

Just to be clear that we are only discussing a BMS shut down of a lithium camper battery.

 

[RAM5500 CAMPERTHING]

I just looked up the LifeBlue website and they claim they are one of the only LiFePo batteries that don't need a DC-DC charger and don't require solar and converters set to a "Lithium" setting as they are a "true" drop in replacement.

Frequently Asked Questions | LiFeBlue RV Boat Lithium Ion Bluetooth Battery

FAQ’s for LiFeBlue Battery. The Smartest Bluetooth Lithium Ion Battery for RV and Marine Use!

www.lifebluebattery.com

I've used Sterling B2B chargers with another brand of Lithium battery and it worked great. However, LifeBlue doesn't require them and allows direct alternator charging that worth considering. Maybe this is only in their current BMS versions. I don't think it would hurt if the Sterling was installed too, but it may be unnecessary. Do you have any insights?

I dont have any insights into that claim. But what i do know, is there is no way of monitoring or limiting the charge if its direct...

It also says the battery is rated to a max charge rate of 150 amps or so

If my dual alternators are capable of 440amps and the Lifeblue SOC is down to 50%, i cant imagine that ending well for either the battery or the alternator without something limiting it

 

[Rando]

Worth noting in this:

-- REDARC, and, I suspect, Sterling Power, require that their B2B be fed from a battery, typically a lead acid starter battery. So the loss of the second (camper/house) battery would be on the other side of the B2B.
-- Neither will provide any protection to the alternator from a problem with the starter battery, which is directly connected to the alternator.

Just to be clear that we are only discussing a BMS shut down of a lithium camper battery.

If the B2B is fed from a battery, there is no issue from a 'load dump' as the battery up stream of the B2B is still in the circuit and will provide plenty of capacitance to the circuit. In the case where you have a LiFePO4 in parallel (and/or through an isolator) with the starting battery, there is also no issue as the starting battery is still in the circuit if the BMS shuts off.

The situation where you have an LiFePO4 charged on its own alternator, as the OP described, is where there is a potential for a problem. In this case I am not sure a B2B would help, and by the statement from Sterling and REDARC, it sounds like they wouldn't help, which is why they require a battery on the input side.

 

[Rando]

I dont have any insights into that claim. But what i do know, is there is no way of monitoring or limiting the charge if its direct...

It also says the battery is rated to a max charge rate of 150 amps or so

If my dual alternators are capable of 440amps and the Lifeblue SOC is down to 50%, i cant imagine that ending well for either the battery or the alternator without something limiting it

Doesn't the LiFeBlue provide built in current monitoring (not control)? So you would know how much current it is drawing, and it will also shutdown charging if it is too high. To reduce the current, you can either dial down the alternator voltage, or add some resistance between the alternator and battery.

 

[ScottPC]

I dont have any insights into that claim. But what i do know, is there is no way of monitoring or limiting the charge if its direct...

It also says the battery is rated to a max charge rate of 150 amps or so

If my dual alternators are capable of 440amps and the Lifeblue SOC is down to 50%, i cant imagine that ending well for either the battery or the alternator without something limiting it

I'd be inclined to go the B2B route and not leave it to the BMS. B2Bs seem really useful for the Smart Alternators, particularly.

 

[DiploStrat]

If the B2B is fed from a battery, there is no issue from a 'load dump' as the battery up stream of the B2B is still in the circuit and will provide plenty of capacitance to the circuit. In the case where you have a LiFePO4 in parallel (and/or through an isolator) with the starting battery, there is also no issue as the starting battery is still in the circuit if the BMS shuts off.

The situation where you have an LiFePO4 charged on its own alternator, as the OP described, is where there is a potential for a problem. In this case I am not sure a B2B would help, and by the statement from Sterling and REDARC, it sounds like they wouldn't help, which is why they require a battery on the input side.

We are talking past each a bit. The point is that a B2B, connected to your starter battery is an alternative to a dedicated alternator for the camper battery.

 

[RAM5500 CAMPERTHING]

Doesn't the LiFeBlue provide built in current monitoring (not control)? So you would know how much current it is drawing, and it will also shutdown charging if it is too high. To reduce the current, you can either dial down the alternator voltage, or add some resistance between the alternator and battery.

Technically yes, but their app is HORRIBLE

 

[RAM5500 CAMPERTHING]

I'd be inclined to go the B2B route and not leave it to the BMS. B2Bs seem really useful for the Smart Alternators, particularly.

Agreed