DC to DC Charger?

[prevostart]

I'm curious about this the LifePO4 battery manuals say to use a DC to DC charger to charge LifePO4 batteries driving down the road, They say they need to charge at 14.6 volts plus or minus .4 volts that would be low at 14.2 volts from your alternator diving down the road, When I'm driving down the road my DN50 alternator is charging at 14 volts down the road and it has 275 amps, So anyone using a DC to DC charger driving down the road??

[Isaac-1]

Yes, I am using a Renogy DCC50S combination DC-DC charger and MPPT solar controller (up to 50 amp when used with alternator alone, when in solar mode with solar input (daytime), alternator output is limited to 25A), it will both boost and reduce voltage as needed and has settings for both custom battery charging voltages and a stock LiFePo4 profile. (custom settings requires a separately sold Bluetooth dongle) Solar MPPT is limited to 25V VOC, so can't be used with larger residential style panels. I installed it about 3 years ago, there may be better options out there now.

[SafariBen]

Two reasons to use a DC to DC converter with LiFePO4, first is to limit charge current to protect the alternator, possibly an overblown concern, or at least there are other ways to do that easily. Second is to get a full balancing charge for long battery life. The most specific info I've found was balancing should be done "every few months", so NOT a big concern as long as you have a way to balance the battery by fully charging it at some point. For most, solar would do that. OR a smaller plug in Li charger could do that.

[d23haynes57]

Originally Posted by prevostart

I'm curious about this the LifePO4 battery manuals say to use a DC to DC charger to charge LifePO4 batteries driving down the road, They say they need to charge at 14.6 volts plus or minus .4 volts that would be low at 14.2 volts from your alternator diving down the road, When I'm driving down the road my DN50 alternator is charging at 14 volts down the road and it has 275 amps, So anyone using a DC to DC charger driving down the road??

The need is dependent on battery capacity versus alternator capacity. Having a Prevost you likely have some capacity. Lithium will take current at anything above 13.8 volts slowing down as the battery voltage increases. They do not need to be fully charged every day. My 270 A/H alternator with the Duvac Diode battery isolation system is supporting 900 A/H lithium just fine. I get peak charge rate up to 160 amps, quickly tapering to ~120 and staying there until the bank gets to ~90%. Then at full charge the internal BMS turns charging off. Ido does not have a residential fridge. I do have hydronic heat which is a power hog. Without AC I go 3- or 4-days winter camping running heat, using the microwave and coffee maker and watching TV.

[prevostart]

Thanks' for the reply, I have 1120 amp hours, four 280 amp hour batteries, wired with bus bars, I think I over did it on the batteries,The reason I bought four is that it had four Lifeline AGM batteries 255 amp hours each so I had 1020 amp hours,From Royale which was a part of Monaco it came with five lifelines 1275 amp hours, some one had removed one of the batteries before we owned it, Most Prevost are all electric no propane. I did not realize the difference with LifePO4 batteries. I'm not going to get rid of batteries so I will make it work, LiTime recommends balancing every 6 to 8 months, I have a LITime 40 amp Lithium charger but how Important is balancing? Right now we have not been using bus for last couple months, Trip in June and August will be dry camping. Coming back from AZ in February drove from Buckeye to Bakersfield in 1 day then another 120 miles home the next day, The volt meter hung right at 14volts seemed fine. When we got home the Victron said 100 percent. Since then I've done testing and put some major loads on batteries and SOC was all over the place so I Charged each battery to 100 percent 1 at a time I added Bus Bars let batteries balance for 24 hours, Connected them to Bus and did more testing way way better, Batteries are within a few percent of each other now, But this is the question am I ok charging 1120 amp hours with the DN50 275 amp alternator at 14v. Victron makes a DC to DC charger 40 or 50 amp can't remember, But I think they have to charge a lead acid or AGM 1st then that battery then charges Lithium batteries, Sounds complicated??? You guys sound like you know a lot about the Lithium set ups, Couple of my bus buddies have almost the same set up as me and it seems to work, But if I can make it better I'm all in, I started with a LITime DC to DC charger and I sent it back I did not like it,got to complicated, LITime support is not the greatest so I've been watching a lot of YouTube Videos. I would have liked Battleborn batteries but out of my price range so LiTime batteries have a lot of bang for the buck so I will figure it out hopefully with some help from you guys Thanks'

[d23haynes57]

50 amps into an 1,150 A/H bank puts you at a 23 hour rate. Just not enough. It may not even keep up with a refrigerator.
If you already had a 1,275 A/H AGM bank being charged by the 275 A/H alternator, plus other possible loads current control was already in place. Those controls may as simple as limited wire size. Also, was charging controlled by relay or diode block. Diode blocks do provide some Lithium advantages.
The real difference in current draw from a fixed voltage source such as alternator is really only about 10% of the rated A/H capacity. I would suggest do some testing and see what you have. Another option is to add some relay controls splitting the bank and charging the batteries in groups.

[SafariBen]

I agree to do some testing. I would not separate them into groups and charge separately, the problem is reconnecting them can cause huge current spikes. Connect them only fully charged, or through a resistor until equalized, then leave them connected, that's what I've read at least. You could set up a relay with a shorter fatter charge wire on a relay and a longer thinner charge wire for continuous use, so that the longer thinner wire with 0.5-1V drop under your max desired current. Another way would be a relay on the short fat charge wire, but bypassed around the relay with a longer thinner wire, so the relay on would allow no resistance charging, and the relay off would allow a charging with a bit of resistance to limit current.

[Isaac-1]

I agree one thing you NEVER want to do is connect different LiFePo4 batteries when they are not perfectly balanced with each other, this is due to the potential for huge currents due to their very low internal resistance.

[twinboat]

Originally Posted by Isaac-1

I agree one thing you NEVER want to do is connect different LiFePo4 batteries when they are not perfectly balanced with each other, this is due to the potential for huge currents due to their very low internal resistance.

I would think that they built in BMS would prevent huge current movements. Most are limited to 100 amps or do.

[shootist]

The charge voltage isn't critical, the cut off when they are 100% is very critical. 3.5 volts per cell is 100%. They will almost immediately drop to 3.33 volts per cell with little to no drawdown. Anything higher you are just wasting energy and possibly causing damage to the batteries. I've seen a demonstration of 170 volts charging a 12v lifepo4 with no shorting or damage. So they are very tolerant as far as charge voltage. The only prevost I've been under the hood on had 2 alternators. If you have really run your batteries down and see really high charging current you might want to cut the alternator off for a cooldown. Most are rated at 70% duty cycle at 100% output, 7 minutes on, 3 minutes off. If you drain your batteries down a lot and charge to 80% then bottom balance them. It's a
Near permanent solution, rarely requiring rebalancing. If you like them topped off then top balancing is better. Only problem is with lithium the weakest cells dump their amps first. A bms is only required for the normal person who doesn't want to manually manage the setup. For me it would be forgetting to manually manage it.

[d23haynes57]

Originally Posted by twinboat

I would think that they built in BMS would prevent huge current movements. Most are limited to 100 amps or do.

The BMS current limiting is not an instantaneous device like a fast blow fuse and there is not like a current limiting resister. However, if you have correct equipment, it can be done.
For manually connecting make a first connection using some 12-volt lightbulbs and wait for the voltages to equalize a bit. I do this when powering up inverters and other capacitor-based power supplies to avoid that inrush arc.

[Isaac-1]

Originally Posted by twinboat

I would think that they built in BMS would prevent huge current movements. Most are limited to 100 amps or do.

The BMS does NOTHING to regulate current flow, they are an on/off switch they either allow current to flow or turn it off.

[prevostart]

Hey guys my electrical skills and mechanical skills are pretty good but these batteries and what you guys are talking baffle my brain If you guys might call me and explain all this to me would be way easier then reading your post I really appreciate your help and post. Art (805) 712-9449 Thanks' I've been a Aqua Hot tech for 14 years and RV repair, replaced a lot of batteries but not Lithium so I do have skills but lithium is new to me. I Retired last years.

[d23haynes57]

Originally Posted by Isaac-1

The BMS does NOTHING to regulate current flow, they are an on/off switch they either allow current to flow or turn it off.

Correct. That includes the Green Colored LiBIM.