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Old 17-08-2022, 06:33   #1
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Cell based lithium charging why not.

I was looking at my system design for my boats Li solution.

Why has the industry not considered isolated dc dc chargers to do cell level charging. On a 3P4s system this is just four dc dc isolated chargers each handling a series string , the cell monitoring could be added as well to the electronics.

This removes any balancing circuits and allows full current balancing in effect as each parallel bank is simply charged to the cutoff point.

It also means ALL dc sources then go through this charging system , so the “ BMS” by default controls all sources and the settings in the source make little difference , ie you just need you mppt , battery charger , alternator to be in a constant voltage power supply mode.

How come this approach doesn’t seem to be offered.
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Old 17-08-2022, 06:56   #2
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Re: Cell based lithium charging why not.

Complexity, added points of failure, no ROI...

But I agree with the direction you're going. I still don't completely understand why there apparently aren't any single-vendor boat DC solutions, where everything (batteries, chargers, inverters, monitors) is designed and sized to work together efficiently. Instead it seems that every owner has to re-invent the wheel and piece together their own system from different vendors.

If I can blue-sky for a moment, the ideal Li battery would actually be a self-contained system. In addition to the battery cells, it has built-in overcurrent protection, integral per-cell charging with a separate charge voltage input, and integrated monitoring (V, A, temp, cell-testing) and control with a data port.

Adding more batteries would just involve paralleling the DC in and DC out terminals, and daisy-chaining the data ports.
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Old 17-08-2022, 07:16   #3
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Re: Cell based lithium charging why not.

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Originally Posted by Lake-Effect View Post
I still don't completely understand why there apparently aren't any single-vendor boat DC solutions, where everything (batteries, chargers, inverters, monitors) is designed and sized to work together efficiently. Instead it seems that every owner has to re-invent the wheel and piece together their own system from different vendors.
Cost, people always buy cheap.

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Old 17-08-2022, 10:49   #4
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Re: Cell based lithium charging why not.

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Originally Posted by Pete7 View Post
Cost, people always buy cheap.



Pete


In a 12v system , it’s a maximum of 4 dc dc chargers. On a 300aH battery , that’s say 10a per parallel cells set. So typically 200 watts solar 30-40 from the alternator or similar from the mains charger.

Hence 4 10A cell chargers. Dc dc converters of that size are cheap as chips. Then add a monitor control circuit to each dc dc unit to handheld charge termination. Temp , lvc , HVC.

Does away with need for external alternator controller , use cheap smps ac dc mains power supplies instead of 3 stage chargers ,simplifies mppt etc.
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Old 17-08-2022, 11:34   #5
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Re: Cell based lithium charging why not.

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Complexity, added points of failure, no ROI...
here is a conceptual diagram for a 12v nominal, 300Ah battery with 10A per parallel cell set charging ( ie 40A max overall )



its actually much less complex as all the charge source controllers are now simple CV control , ie mains charger is a cheap SMPS, standard stock alternator regulator is fine, no need for sophisticated charge controllers dotted around the boat all having to be setup and all having to separately determine tail current to accurately stop charging

hence cheaper overall , no more complex that existing systems

Integrated cell monitor

no balancing circuits needed or charge delays while active or passive diversion balancing is going on

scale up by either bigger DC DC or paralleling DC DC units
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Old 17-08-2022, 12:30   #6
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Re: Cell based lithium charging why not.

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Cost, people always buy cheap.
Valid point. But there's not much 'cheap' about Li marine batteries, yet.

At the manufacturing level, if boat makers could easily choose the right-sized integrated DC system based only on anticipated load, and that there's no engineering required to have the chosen DC solution "just work", then it could find its way into the market via new boats.

When things are "commoditized", they usually become more affordable.
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Old 18-08-2022, 04:54   #7
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Re: Cell based lithium charging why not.

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Valid point. But there's not much 'cheap' about Li marine batteries, yet.

At the manufacturing level, if boat makers could easily choose the right-sized integrated DC system based only on anticipated load, and that there's no engineering required to have the chosen DC solution "just work", then it could find its way into the market via new boats.

When things are "commoditized", they usually become more affordable.


For lithium currently the user is faced with a “ mess” even new equipment is designed to network together as a system. It’s like helm electronics from the 80s

We really need a whole range of suitable interconnected mains chargers , mppt, alternator regulators and BMS projects. Victron is probably out in front ( their open Ve.CAN project is great ) with Ve. Smart and Cerbo/Venus products. But even here there’s various legacy issues and not everything interworks with everything
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Old 18-08-2022, 08:20   #8
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Re: Cell based lithium charging why not.

This design is called Node Charging.

Each cell/group gets its own charge regulator.

There is no balancing, other than each is accurate and set to the same charge termination algo/setpoints.

Very common in the RC Hobby charger world, but trending down.

OTS units would be just fine for the last part of the charge cycle at low amps, analogous to the balancing stage, maybe just for periodic maintenance on shore power, not suitable for Bulk/CC early high current stage.

Not expensive either.

The same could be done with big (say 0.4C current rate) PSUs / DCDC buck converters, each fitted with an HVC that can be custom adjusted with precision. These need to be isolated circuits with adjustable current limiting. Not cheap if reliability is desired, if going cheap-Chinese keep lots of spares and watch the charging process like a hawk, overtemp cutoffs a must.

A protective-only BMS should also still be fitted as redundant LVC/HVC at the system level isolating the bank based on cell/group level voltages.
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Old 18-08-2022, 15:17   #9
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Re: Cell based lithium charging why not.

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Originally Posted by PaulCrawhorn View Post
This design is called Node Charging.

Each cell/group gets its own charge regulator.

There is no balancing, other than each is accurate and set to the same charge termination algo/setpoints.

Very common in the RC Hobby charger world, but trending down.

OTS units would be just fine for the last part of the charge cycle at low amps, analogous to the balancing stage, maybe just for periodic maintenance on shore power, not suitable for Bulk/CC early high current stage.

Not expensive either.

The same could be done with big (say 0.4C current rate) PSUs / DCDC buck converters, each fitted with an HVC that can be custom adjusted with precision. These need to be isolated circuits with adjustable current limiting. Not cheap if reliability is desired, if going cheap-Chinese keep lots of spares and watch the charging process like a hawk, overtemp cutoffs a must.

A protective-only BMS should also still be fitted as redundant LVC/HVC at the system level isolating the bank based on cell/group level voltages.


Cant see the point of adding a second BMS. there’s one in the cell chargers that’s effectively a full bms. Adding another offers nothing.
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Old 18-08-2022, 17:40   #10
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Re: Cell based lithium charging why not.

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Cant see the point of adding a second BMS. there’s one in the cell chargers that’s effectively a full bms. Adding another offers nothing.
No there are many many potential protections that a BMS can offer not included in any charger.

In fact better BMSs control the charging process via signaling to the charger.

Chargers never have undertemp cutoff, nor LVC for example.

And BMS on the cheap end are never primary controls, should just be for redundancy, pack protection when the primary circuit fails.
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Old 18-08-2022, 20:44   #11
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Re: Cell based lithium charging why not.

Active balancing boards are cheap and effective ! Used them for years .
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Old 18-08-2022, 20:55   #12
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Re: Cell based lithium charging why not.

For small systems looking great

You can even combine charging methods: do 80% of charging for all cells together, plus 20% in-line cell based charging. When these communicate, they can go slower when their cell is ahead.

For larger systems it becomes complex… we put 140A through 8 cells in series.
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Old 20-08-2022, 13:48   #13
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Re: Cell based lithium charging why not.

I think it’s fine where there is a fixed charge rate. But in a system where there can be a wide variety of charge sources and widely varying power capacity, the node chargers would either be the limiting factor for charge rate, or need to be very high capacity.
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Old 21-08-2022, 13:45   #14
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Re: Cell based lithium charging why not.

This an interesting idea. The built in charger bms could provide the necessary advanced alert and shutdown contacts to meet ABYC.
The chargers would have to communicate and one probably be master.

I dont believe a wakespeed reg would be necessary.

The only question I would have, isn't 10a capacity a very low C rate? I would want more like .5C.
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Old 23-08-2022, 21:36   #15
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Re: Cell based lithium charging why not.

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I think it’s fine where there is a fixed charge rate. But in a system where there can be a wide variety of charge sources and widely varying power capacity, the node chargers would either be the limiting factor for charge rate, or need to be very high capacity.
All the node chargers I'm aware of are DC-DC, and certainly that is standard for the balancing charger world.

Most can accept 12V but for maximum output many require 24V or higher.
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