Home Built Hybrid EV Battery Charge current limiting

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I have a hybrid EV that I'm working on which uses 2 48V DC motors with a maximum load of 300 Amps each, for a total maximum load of 600A. I have 2 alternators running off a gas engine generating 125 amps each wired in parallel for a total static current of 250A. I want to use a battery system to pull the remaining 350A in short bursts of power. I've sourced a 100Ah 48V Li-ion pack in order to supply the necessary current.

My issue that I run into is that I want the alternators to charge the batteries when they're not being fully drawn, but the charge current for the batteries shouldn't exceed .5C, or 50A, so it's not an easy alternator -> battery -> motor-controller setup.

So, what do I need in order to connect all these components together in order to have the system I'm looking for?

Best Answer

Many vehicle designs have a single bus that carries the current to the three systems. Using mosfets, or in the simple case - diodes, you can switch current onto the bus, or shunt it off into the system you're dealing with. Typically in a dumb system the bus is kept at voltage within a certain range, but that voltage is allowed to move up and down within that range, and each system acts differently based on the bus voltage.

Taking each system one at a time:

Alternators

The alternators attempt to keep the voltage at, say, 52V. As long as it's at or near 52V, they regulate their current output so the bus stays between, say, 50 and 52. When the voltage is below 50V, they run full tilt at 250A to get it back up there. That's their only job, since they never take current from the bus.

Motors

The motors don't care about the bus voltage. The throttle shunts current from the bus to the motors, and the bus voltage drops if the other systems aren't adding current quickly enough.

Batteries

The batteries are the more complicated system. If the bus voltage is 50V or higher, they take current from the bus, and charge the batteries. Just activate the 0.5A charger when the bus voltage is above 50V. 50V is the signal that the alternators have excess capacity, and the batteries can charge. They do not put any current onto the bus.

If the voltage is 48V to 50V, the batteries disconnect themselves from the bus if they aren't already supplying current. If they are supplying current, they continue to do so, trying to keep the bus voltage above 48V, but below 50V.

If the voltage is under 48V, the batteries start trying to force current onto the bus, keeping it between 48V and 50V.

The batteries only do this, of course, until they run to about 50% of their capacity, then they stop supplying current to extend the life of the batteries.

Result

When the motors draw no current, the alternators are running on a light load, charging the batteries.

When the motors are drawing 250A or less, the alternators are running at full load, and the batteries are disconnected.

When the motors are drawing over 250A, the alternators are running at full load, and the batteries are fully connected to the bus, pushing as much current as they have left onto the bus.

Reality

This system has its faults. If the battery-->bus or alternator-->bus regulators aren't happy with each other, you'll end up with system oscillation which might not be easily resolved.

Most modern systems use active elements and a single bus controller that tells each system what to do. This controller knows how much current the motor is commanding, and tells each subsystem how much current to supply, while monitoring other bus and system vitals to prevent overvoltage, overcurrent, and overtemperature violations.

But with careful design and tuning, the dumb system can work, and works well. An advantage is that the system can handle more devices either providing or using current without having to specifically communicate with a central controller. If you add solar panels, or a cabin cooling system, they hook directly to the bus with a little logic that turns them on or off depending on the bus voltage. On a central bus system you will likely have to reprogram the central controller and may even have to have these devices communicate with it so it doesn't accidentally discharge the batteries when it believes it is in a charge state, for instance.

90.5% @0–100% SoC

for electrolyte type A

89.1% @0–100% SoC

for electrolyte type B

I read reverse calculated this from graphs in a non-public measurement report where 2 different types of battery electrolyte (solid/gel) technologies were compared for a 0 to 100% SoC (state-of-charge).

Battery measurements were taken by a technical university, at 21ºC room temperature during 1 month, the year 2013, comparing the results from 150-180 discharge/charge cycles. Batteries under test were all in the range between 5 and 10Ah.

charging curve: CC-CV
charge to discharge point: V = 14.375V and I < 0.30A
discharge to charge point: 0% SoC
resting period: not specified
0% SoC was defined as discharge voltage reached 10.8V (1.8V per cell)
100 SoC was defined as charge voltage reached 14.375V (2.396V per cell)
charge was done at a rate of 0.3C (+/- 0.05C)
discharge was done at a rate of approximately 0.1C (fixed resistor was used)

The result between charging and discharge power was (remarkably) about 1.6Wh for both battery types, where one battery had a remaining capacity of approximately 40Wh and the other 20Wh.

Some data read back from the graph (resulting in lower accuracy) for electrolyte technology A:

Cycle   Charge [Wh]   Discharge [Wh]
1       100           91
2       100           88
3       100           86
4        97           81
5        92           76
6        86           73
7        81           69
8        78           67
9        74           64
10       73           63
11       70           61
12       69           59
13       65           56
14       63           55
15       61           52

136      44           40
137      44           40
138      44           40
139      45           40
140      45           40
141      43           40
142      44           40
143      45           40
144      44           40
145      44           40

According to my opinion the last measurements are more significant than the first measurements due to the initial formation process. Average last 10 cycles power charged into the battery is 44.2Wh. Average last 10 cycles power discharged from the battery is 40.0Wh.

Some data read back from the graph (human graph point via line to value translation) for electrolyte technology B:

Cycle   Charge [Wh]   Discharge [Wh]
1        85           75
2        83           72
3        80           70
4        78           69
5        76           67
6        76           67
7        75           67
8        75           67
9        74           66
10       74           66
11       73           65
12       73           66
13       72           66
14       72           65
15       73           64

136      25           22
137      25           22
138      25           22
139      24           22
140      25           22
141      24           22
142      25           22
143      25           22
144      25           22
145      24           22

Average power charged into the battery during last 10 cycles is 24.7Wh. Average power discharged from the battery is 22.0Wh.

Electronic – Current limiter – charging battery from battery

This is not easy as a linear limiter and better with a PWM limiter and LC filter.

If the 12Vaux is fully discharged and requires (14.2-11.2)V*20A=60W series load dump . That is a lot of heat before the Imax reduces to a CV equal voltage to the alternator.

If one considers a light bulb a good solution , this is effectively a PTC almost constant current source when used in the 0 to 10% voltage range defined by DCR which is 10% of rated voltage R. e.g. 12V/20A where filament resistance drops to 10% of DCR at rated voltage at 3200'K Thus 3V*20A=60W , with a DCR of 0.15 ohms then at 10V and 10% DCR the bulb may be something like 30V 1.5Ohms or 45W which is really not a practical value. It might be something like to 12V 70W car headlamps in series/parallel unless you want an AUX light when charging.

So ok in lower currents, but not 20A. This is just a ballpark estimate, not a rigorous calc.

A better solution is PWM with a series choke rated for 20A such as those air coils found in ATX PSU's and a fast switch rate with a MOSFET switch rated for 50A ( hi side or low side. ) THe battery acts as a 10kfarad capacitor with some ESR from 5 mOhm to 1 Ohm when dead. but an RF cap will reduce EMI.

Another way is to just use a fixed 150 mOhm 60W heater wire (NiChrome) and use that to keep your coffee cup warm ;) Or use a Cap Pulse discharger rated for high RMS ripple current ( several large plastic caps in parallel) This is essentially an active SMPS current limiter with a 0.1V max drop at CV mode at 14.2V or 0.1V/20A = 5 milliohm (MOSFET + choke DCR)

I said to do this right is not easy and stay within limits.

Ultimately the simplest solution is get a bigger battery rated for the current of the alternator, but then you may end up blowing your Alternator diode bridge with both batteries are weak and drawing max current of alternator for long periods at 180'C junction temp.

You can look at NTC disc surge limiters, but they cannot protect a battery as they are designed to protect perhaps only 0.5Farad and not a battery with 10k~100K Farads. To understand read , https://www.digikey.com/en/ptm/a/amphenol-advanced-sensors/cl-series-inrush-current-limiters/tutorial. To dump 40 Watts of heat it either has to run extremely hot or be big like headlamp bulbs ( which will get hot and must be sealed from moisture).