VSR instead of B2B in LiFePo / lead acid hybrid system?

fredrussell

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I’m just musing here so don’t all shoot me down too violently! It’s an area I know very little about, obviously.

My van needs a new leisure battery. It currently has two lead acid batteries - starter and 100 a/h house. They are linked by a 40 amp VSR. There is a 40 amp fuse at output of VSR (between VSR and house battery). Van has a 90 amp alternator.

If I was to replace house battery with 50 a/h LiFePo battery, would the VSR limit the amps being ‘taken’ by an empty house battery be limited to 40?

…and would the LiFePo BMS stop any harmful ampage draw on the alternator? (let’s assume for arguments sake it’s a decent BMS).

What’s confusing me is that if a B2B is there to stop the LiFePo drawing to much amps from the alternator, would a 40amp VSR (with 40 amp fuse) achieve the same thing?
 

kwb78

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No it wouldn’t. A VSR is just a connection which is conditional on the voltage of the supply battery/alternator. When the voltage increases above the threshold, the VSR connects and current will flow to the leisure battery. A LiFePo4 battery has almost no internal resistance, so will draw as much current as a source will supply. It will immediately blow the 40A fuse on the VSR, which will protect the alternator but obviously will not allow charging.

There are two problems using an alternator to directly charge an LFP battery. First is the ability of the alternator to supply the current the battery is pulling. It may be able to supply 90A, but can it do that continuously without burning out the regulator because of overheating? The second is the case where the BMS disconnects the battery suddenly due to over voltage when the battery is full. This can cause a voltage spike at the alternator that can kill the diode pack or regulator. That shouldn’t actually be an issue if a lead acid battery is in parallel as that will prevent the voltage spiking.

The reason a VSR works ok for lead acid batteries is that their internal resistance is higher, and limits the current they draw. It might pull near the alternator’s maximum output initially if heavily discharged, but that will quickly reduce as the battery charges with the current tailing off significantly when the battery comes up to full charge. A LFP battery won’t do that, it will pull the full current regardless of charge until it reaches full capacity.

The other issue with directly charging an LFP battery is that the charge is uncontrolled, and it relies on the BMS to cut off charging when full. That’s not good practice because it’s a single point of failure - ideally a charger will stop at a pre-defined voltage set a bit below the BMS cut off.
 

Trident

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Connect the alternator to the LA battery - then anything going to the Lithium is "flowed" through that and the BMS switching off will not hurt the alternator.

You can use a VSR with LifePo4 but it will not charge them necessarily at the right rate (its probably ok if you have solar on the van too to mostly keep the Lithium topped up and then VSR is a back up when motoring ) - you will need to know the maximum charge rate of the BMS on the lithium - some are 1c so on a 50AH that would be 50 amp and so a 60 amp fuse to the vSR will allow it to charge at full rate and still not stress a 90amp alternator . Most VSRs are rated at 100+ amps so that will always be ok. However, some Lithium batteries (cheaper generally) have a 0.5C charge rate on the BMS so may only allow 25a, whilst very good ones may allow more than 1C so could pull more and blow the fuse. Pick a good battery, and check its charge rate.

One thing to note is VSR works both ways and the working voltage is ofter 13.2 which is the normal voltage of a Lithium from 90% down to 10% so the lithium will try to back charge the lead acid when the engine is off. The lead acid should always be full but ...

All in all given the relatively low cost of DCDC (B2B) chargers now its probably just better to get one - A Renogy 40DCD is just £129 and the 50AH Renogy LifePo4 is on sale at £239 - with 5% off your first purchase so £350 for the pair which is pretty cheap for what you achieve :)
 

geem

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If I were yuo I would install a 100Ah LiFePO4 Battery and use a Sterling or Victron B2B to charge it. You can set the correct charging parameters in the B2B so you are not relying on the BMS to go in to cut off mode. The BMS cutting off the supply should be seen as a safety device in the event of over charging, not a device to control charging. A 100Ah LiFePO4 Battery would be best used between 20 and 90% of its capacity. There is no need to keep it fully charger. It is best stored at 50% of its capacity when not being used
 

geem

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I would look at Fogstar batteries in the UK. They build them from good cells and BMS. If I was buying a LiFePO4 drop in Battery, they would be my first choice.
Rod Collins of marinehowto.com did an excellent detailed article on drop in batteries
Drop-In LiFePo4- Be an Educated Consumer
He actually says stay clear of Renogy kit. More failures than any other equipment in the USA
 

fredrussell

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Connect the alternator to the LA battery - then anything going to the Lithium is "flowed" through that and the BMS switching off will not hurt the alternator.

You can use a VSR with LifePo4 but it will not charge them necessarily at the right rate (its probably ok if you have solar on the van too to mostly keep the Lithium topped up and then VSR is a back up when motoring ) - you will need to know the maximum charge rate of the BMS on the lithium -
My van does have an 80 watt solar panel on roof, apols- should have mentioned that.

Thanks to all - clears that up.
 

Trident

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I would look at Fogstar batteries in the UK. They build them from good cells and BMS. If I was buying a LiFePO4 drop in Battery, they would be my first choice.
Rod Collins of marinehowto.com did an excellent detailed article on drop in batteries
Drop-In LiFePo4- Be an Educated Consumer
He actually says stay clear of Renogy kit. More failures than any other equipment in the USA
He did write that several years ago and before Renogy had brought out their batteries and the article is now rather dated in many other respects too. Fogstar do seem to be very good and I have an account with them too and have found them great to deal with other than changing prices a lot but they have been hugely overwhelmed by orders so thats not a surprise - the trouble is getting them as they're now taking deposits for end of June delivery at the earliest - partly because they they have been so successful with putting a good BMS and agreed A sells together at a great cost.
 

geem

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He did write that several years ago and before Renogy had brought out their batteries and the article is now rather dated in many other respects too. Fogstar do seem to be very good and I have an account with them too and have found them great to deal with other than changing prices a lot but they have been hugely overwhelmed by orders so thats not a surprise - the trouble is getting them as they're now taking deposits for end of June delivery at the earliest - partly because they they have been so successful with putting a good BMS and agreed A sells together at a great cost.
Yep, I built my own battery. Cheaper than any good drop in and better suited to the Tropics as the bms can be kept off the cells to reduce cell hot spots and the whole lot properly ventilated. I see the attraction of 'drop in' but not for Summer Tropical temperatures. Even worse in the Med where the summer heatwaves that now seem to be quite common would potentially heat stress the batteries. More likely to see BMS drop outs on high FET temperature when running large loads
 

William_H

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Quote kwb78

There are two problems using an alternator to directly charge an LFP battery. First is the ability of the alternator to supply the current the battery is pulling. It may be able to supply 90A, but can it do that continuously without burning out the regulator because of overheating? The second is the case where the BMS disconnects the battery suddenly due to over voltage when the battery is full. This can cause a voltage spike at the alternator that can kill the diode pack or regulator. That shouldn’t actually be an issue if a lead acid battery is in parallel as that will prevent the voltage spiking.

An alternator will supply current depending on demand and also depending on RPM and rating of the alternator. 80Amp alternator would only provide that at high RPM and even then at reduced output voltage. Getting max current out of an alternator can cause overheating. (will not damage the regulator which simply provides max curren to the field coils) Due to internal resistance the output voltage of the alternator tends to fall at high current after regulator gets to max field current. Hence I think that with a LA battery on alternator output line the VSR can only deliver to Lithium battery max 14v which will decline as lithium takes charge current. If lithium takes too much current charging then a low value resistor in the feed line will limit current by reducing voltage from supply. So my guess to be proven in practice is that charging lithium from alternator LA battery VSR will be OK. Monitor alternator charge current to avert overheating of alternator. ol'will
 

jakew009

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An alternator will supply current depending on demand and also depending on RPM and rating of the alternator. 80Amp alternator would only provide that at high RPM and even then at reduced output voltage. Getting max current out of an alternator can cause overheating. (will not damage the regulator which simply provides max curren to the field coils) Due to internal resistance the output voltage of the alternator tends to fall at high current after regulator gets to max field current. Hence I think that with a LA battery on alternator output line the VSR can only deliver to Lithium battery max 14v which will decline as lithium takes charge current. If lithium takes too much current charging then a low value resistor in the feed line will limit current by reducing voltage from supply. So my guess to be proven in practice is that charging lithium from alternator LA battery VSR will be OK. Monitor alternator charge current to avert overheating of alternator. ol'will

You really have no idea what you are talking about. What resistor do you recommend that can handle an 80A load?
 

kwb78

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Hence I think that with a LA battery on alternator output line the VSR can only deliver to Lithium battery max 14v which will decline as lithium takes charge current.
No, it won’t - the Lithium battery will not reduce its charge current until it is full. The charge curve is very flat, and the resistance is very low. They do not behave like lead acid batteries in that regard. What will happen is that the battery will pull as much as the alternator can supply and continue to do so until until the last 1% or so of charge at which point the battery voltage does start to rise.

Most alternators can’t deal with that type of load for long periods unless they run at high speed because they are not sufficiently cooled. On boats this is exacerbated because engines are usually in enclosed spaces that aren’t necessarily well ventilated.

Without some form of external alternator control that can account for speed and temperature, it’s highly likely to be damaged when the engine is running at idle or low revs.

Here is a video by victron that demonstrates the problem:

There is a secondary issue that when fully charged, an LFP battery shouldn’t be held at high charging voltage. Ideally charging should be done at no higher than 3.65V per cell (so 14.V for a 12V system) and that should drop to a lower float voltage once charging is complete so as to maximise battery life.
 

geem

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Quote kwb78

There are two problems using an alternator to directly charge an LFP battery. First is the ability of the alternator to supply the current the battery is pulling. It may be able to supply 90A, but can it do that continuously without burning out the regulator because of overheating? The second is the case where the BMS disconnects the battery suddenly due to over voltage when the battery is full. This can cause a voltage spike at the alternator that can kill the diode pack or regulator. That shouldn’t actually be an issue if a lead acid battery is in parallel as that will prevent the voltage spiking.

An alternator will supply current depending on demand and also depending on RPM and rating of the alternator. 80Amp alternator would only provide that at high RPM and even then at reduced output voltage. Getting max current out of an alternator can cause overheating. (will not damage the regulator which simply provides max curren to the field coils) Due to internal resistance the output voltage of the alternator tends to fall at high current after regulator gets to max field current. Hence I think that with a LA battery on alternator output line the VSR can only deliver to Lithium battery max 14v which will decline as lithium takes charge current. If lithium takes too much current charging then a low value resistor in the feed line will limit current by reducing voltage from supply. So my guess to be proven in practice is that charging lithium from alternator LA battery VSR will be OK. Monitor alternator charge current to avert overheating of alternator. ol'will
Unless the alternator has built in temperature control, the alternator will run at its maximum output for as long as the lithium battery demands it. Typically, the alternator will burn out. The rule of thumb when charging lithium with an alternator is to charge at 50% of the rated output of the alternator. To be able to do this you need a programmable external alternator regulator. Forget what you know about lead. Lithium is totally different. The battery voltage is almost constant all the way to about 95% at which point is will start to rise quickly.
 

William_H

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You really have no idea what you are talking about. What resistor do you recommend that can handle an 80A load?
Likely you are right. However a resistance of .01 ohms at 80 amps will drop the voltage by .8 volt. Dissipating 64 watt. That can be fabricated with a section of ss sheet or wire. Even a robust amp meter shunt might do it. Yes it will get hot. However I would imagine even with charging a lithium battery that a reduction of .8 volt would reduce current significantly. So reducing volt drop and dissipation.
Interesting video. Do all small alternators as fitted to cars and small boat engines have an internal fan? I think not but may be wrong. yes definitely the amount of air moved by a fan is something like the square of speed. So if has a fan yes run alternator at high speed. (smaller pulley on alternator)
Now an alternator will droop in max supplied voltage at greater load. (due to internal resistance) Will this drop in voltage at lower alternator speeds avert overheating. As happens charging LA batteries. Will this volt drop reduce charge current of lithium battery? Certainly not if you are using a B2B charger.(which can use lower voltage to convert to higher voltage fro charging.) All these questions. Yes you are right I don't know what I am talking about. ol'will
 

kwb78

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Most alternators will have a fan. Some are internal and some external. Alternators are not really very efficient, so produce a lot of heat and active cooling is required.

Now an alternator will droop in max supplied voltage at greater load. (due to internal resistance) Will this drop in voltage at lower alternator speeds avert overheating. As happens charging LA batteries. Will this volt drop reduce charge current of lithium battery?

A drop in alternator voltage at high load is not due to internal resistance but because it is saturated and can't supply any more power. The regulator can only supply so much current to the field windings, and once they are at maximum, an unlimited load will pull the voltage down. This will reach an equilibrium and the alternator starts to behave as a constant current source providing its maximum current until the battery voltage comes up and the current starts to drop. Once the battery starts to pull less than the maximum alternator output, the regulator will start to reduce the field current again to prevent it going over voltage. The problem with LFP batteries is that this doesn't happen until the state of charge is approaching 100%. This is the charge curve of a LFP battery:

charge.jpg


This is the charge curve of a lead acid battery:

1685549609778.png

You can see that the lead acid battery also has a bulk charge period where it is in constant current charging, but the difference between it and the LFP battery is that the internal resistance of the lead acid battery is higher to start with, and it also increases during the charge which inherently limits the charging current. The alternator is under high load for a much shorter period, and unless the battery is high capacity and quite flat, that high load is not likely to be continuously at the alternator's rated maximum. The internal resistance of the LFP battery does not change during charging which means it draws as much current at 80% charged as it does at 10% charge so the alternator is worked much harder for a longer period.

Running the alternator at lower speeds will not avert overheating but make it worse. This is because the power dissipated as heat is proportional to the square of the current, not the voltage. The resistance of the alternator windings won't change very much, so an 80A output at idle is going to be producing the same amount of heat as 80A of output at full throttle. The difference is that at idle, there is much less air flowing through the alternator to cool it. Here's quite a good video from Sterling Power that talks about this:

 

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Hi kwb78

Great explanation and kinda what I was expecting when I switched to lifepo4, but I observed in practice a much less dramatic effect.

My agm house battery swapped for 105ah lifepo4 with bms set for 12v low voltage and 14v high volt pack disconnect, which should keep between approx 10% and 90% soc. Stock volvo 115a alternator that regulates at 14.2v. Approx 5m one way 35mm2 cable run between alternator and lifepo4. VSR that connects lifepo4 to sla starter battery to act as a sink for when bms disonnects when charging.

At lowest soc lifepo4 accepts around 70a from alternator with about 13.3 v at the battery terminals, as volts at battery terminal increase the current gradually drops to around 50a at 50% soc and ends up at less than 15a when 14v pack over voltage disconnect occurs.

My conclusion is that the voltage drop along the battery cable is enough to reduce charge acceptance of lifepo4. It definitely charges quicker than agm and the tail off in charge acceptance is much more gradual, but still happens. End result is faster charging but without putting excessive strain on the alternator. I was ready to put in a b2b but decided it was not necessary, every install will be different but it can just work....
 

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Hi kwb78

Great explanation and kinda what I was expecting when I switched to lifepo4, but I observed in practice a much less dramatic effect.

My agm house battery swapped for 105ah lifepo4 with bms set for 12v low voltage and 14v high volt pack disconnect, which should keep between approx 10% and 90% soc. Stock volvo 115a alternator that regulates at 14.2v. Approx 5m one way 35mm2 cable run between alternator and lifepo4. VSR that connects lifepo4 to sla starter battery to act as a sink for when bms disonnects when charging.

At lowest soc lifepo4 accepts around 70a from alternator with about 13.3 v at the battery terminals, as volts at battery terminal increase the current gradually drops to around 50a at 50% soc and ends up at less than 15a when 14v pack over voltage disconnect occurs.

My conclusion is that the voltage drop along the battery cable is enough to reduce charge acceptance of lifepo4. It definitely charges quicker than agm and the tail off in charge acceptance is much more gradual, but still happens. End result is faster charging but without putting excessive strain on the alternator. I was ready to put in a b2b but decided it was not necessary, every install will be different but it can just work....
That's a very small battery. You probably get away with it because you have a relatively large alternator and small battery. My lifePO4 battery is 560Ah in 12v money (24vboat) the amount of amps that battery would pull is considerably more.
LifePO4 has better charge acceptance but in reality some of your alternator output is going to heat the wire.
 

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Thanks kwb78 for the trouble you have gone to, to explain the voltage current regime of Lithium batteries. I don't however understand how a reduction in charging voltage can not limit charging current. Now of course you must have a charging voltage in excess of the inherent cell voltage and as soon as you exceed that charge voltage current can be quite high in a very non linier manner. (a bit like an LED diode) However it still seems to me that reducing charge voltage will reduce charge current and if this is done by series resistance or a droop in alternator voltage then a balance of charge voltage and current must be achieved. ie your graphs show nicely the cell voltage with constant current. I am talking about not constant current. ol'will confused. Perhaps others are also.
 

ckris

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Thanks kwb78 for the trouble you have gone to, to explain the voltage current regime of Lithium batteries. I don't however understand how a reduction in charging voltage can not limit charging current. Now of course you must have a charging voltage in excess of the inherent cell voltage and as soon as you exceed that charge voltage current can be quite high in a very non linier manner. (a bit like an LED diode) However it still seems to me that reducing charge voltage will reduce charge current and if this is done by series resistance or a droop in alternator voltage then a balance of charge voltage and current must be achieved. ie your graphs show nicely the cell voltage with constant current. I am talking about not constant current. ol'will confused. Perhaps others are also.
Exactly what I see happening in practice, with the battery cable voltage drop having same effect as a resistor.
 
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