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Old 03-29-2023, 07:42 AM   #1
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320A Alternator vs Li-BIM 225

Good Morning,

I have had the following posted over on the Forest River Forum under Electrical for the last few days with no response. So I am reposting over here in the hopes I can link up with someone who has the knowledge. Thx.

Long post, thank you for your patience.

I am in the process of upgrading to a LIM 225 BIM and I have a concern ref the alternator capacity vs the BIM circuit, but first some background.


I purchased a used Class C RV, 2017 Dynamax ISATA 4 31DS, and I am in the process of upgrading from the stock Precision Circuits 10033-300D BIM to a new Precision Circuits Li-BIM 225.

The previous owner was an EV specialist and replaced the old 4K generator with a large (780Ah) LiFePO4 battery bank managed by a REC BMS. A large solar setup (1440W) was installed managed by a Magnum Energy PT-100. The inverter/converter was upgraded to a Magnum Energy MSH3012M, rated for 3KW. In addition, he upgraded the alternator on the Ford E450 to one rated for 320A. When I bought the rig he had the Li-BIM 225 sitting on a shelf and I am now installing that system.

The stock Precision Circuit's 10033-300D BIM that is being replaced is, I believe(?), rated for 60A; the circuit was protected by a 150A resettable fuse. The system relay was functioning on engine start.

As per the specifications I have pulled new 2/0 cable and fused both ends with 225A class T fuses to protect the Li BIM 225 and both ends from a cable short. I have also wired in a large capacity On/Off switch (battery switch) on the chassis side of the circuit to allow me to isolate the two battery systems.


Q1: Is there a problem with the design of this circuit (220A) given the capacity of the alternator (320A)?

Q2: And if there is a problem, are there any suggestions on how to manage the current so I do not blow the fuse ($$)?


1) Precision Circuits does not mention alternator capacity in all their documentation I have found so far; not sure if that means it is not a factor or for some other reason. I have emailed Precision Circuits tech support with this design and they said "you should be good". I am worried about 'should be'; I was hoping for a more definitive answer.

2) The upgraded alternator has been working with the old stock BIM (60A & 150A fuse) for at least a year and so far it has not tripped the fuse. However, hearing the relay function on engine start is not a guarantee that the BIM was functioning correctly by fully closing the circuit so I am uncertain if this fact can be used as a benchmark on maximum current flows to date or into the future.

3) Senario - If we boondock and drawdown the battery bank overnight, plus we start moving before the solar has a chance to put a dent into the recharge; I am concerned that once the Li BIM 225 closes the circuit the alternator will see a relatively large voltage difference and 'open the tap' overwhelming the fuse. What I do not have the detailed knowledge for is how the alternator determines output current and if that mechanism would ramp the current up to over 220A given the expected voltage differences between the two 12v systems when the Li bank is well drained.

Thank you for your inputs and time.

Cheers, Ian
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Old 03-29-2023, 08:23 AM   #2
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Scrap the BIM and go with DC to DC chargers.
The BIM cycles the connection on and off, allowing the alternator to cool, not the most effective charging style.

With just the alternator output, you will not fully charge your lithium, only getting to about 80% SOC.

You can parallel DC to DC chargers to get the amp output you want to allow and they, by design, boost the output voltage for full charging.

You do loose bi-directional charging on most DC to DC chargers but I've read that some have that option.
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Old 03-29-2023, 08:53 AM   #3
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I agree with both Precision Circuits and twinboat.

You’ll probably never see 300amps coming out of that Alternator so it should be fine. But Twinboat is right, D2D chargers are a better solution.

I have 400aH of LFP batteries and I removed the BIM entirely and put in a Victron D2D charger. It works great.

Mine is only a 30-amp charger but my LFP batteries are so capable I find I don’t draw all that much power from them-certainly not overnight.

I considered the Li-BIM but like Twinboat didn’t like the idea of the on/off setup not fully charging my batteries. However, with my experience it may have been fine.

I did have to add an Amp-L-Start to keep my chassis battery from running down when parked for more than a few days. The Li-BIM would eliminate that need.

I don’t know if your alternator is regulated or not, most aren’t, but the alternator output is usually governed by the battery/wiring resistance. If your batteries are really depleted (10% SOC) they will accept a lot of current but if not seriously depleted (70%) they won’t take all that much.
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Old 03-29-2023, 10:14 AM   #4
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Totally agree with responses above, thats what I would do If it was my system as well.
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Old 03-29-2023, 09:10 PM   #5
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Another vote here for a DC-DC charger instead of the LiBIM. Charging a LiFePo4 battery is like trying to fill a bottomless pit, your flow limitation when charging with an alternator will be controlled by things wire size, not the internal resistance of the battery bank. A bigger alternator will be maxed out just like the standard alternator, it will just be producing more current and heat while it is switched on by the LiBIM
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Old 03-30-2023, 06:52 AM   #6
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Another vote for Dc to DC charger. We had the LiBim and like others that have had the problem, it stayed latched all the time. The way I noticed it was coach and house batteries had the same voltage.
Installed two Renogy 60 amp DC to DC chargers. I like the Victron but the price is too high.
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Old 03-30-2023, 06:57 AM   #7
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DC-DC vs LiBIM every day of the week...... We have 2 and have done installs with as many as 4. They should all be protected individually, we use the Midnight Solar "Baby Box" and protect both sides (In and Out) and then to a Bus Bar or Lynx Power distribution module.
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Old 03-30-2023, 07:12 AM   #8
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This is an interesting conversation, but I'll add a dumb question because I don't know the answer. First, it sounds as if a LiBIM won't fully charge the battery due to its on-off function. The OP stated that he has 1440 watts of solar. If the solar is configured correctly, wouldn't it aid in topping of the batteries?
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Old 03-30-2023, 07:23 AM   #9
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Lithiums low internal resistance will accept large amounts of current to charge and the BIM does not limit current instead it has duty cycle of 15 minutes on 20 minutes off I believe. This allows the alternator to cool then heat soak rather than maintain constant temperature, should still be effective at protecting the alternator, but the alternators max current will be potentially above the wire and BIM rating, this is also around 0.4C charge rate on that size bank of batteries.

Any alternator that gets above 13.8v will fully charge a LFP battery given enough time, the higher the voltage the faster it will absorb to full. The usual 14.1-14.4v alternator voltage will bring LFP to full very quickly and normally even engage the balancer.

Most alternators are not designed for 100% continuous output which is why the li-BIM is used or a current limiting device like a DC-DC charger or a second alternator with regulator.

In your case the main issue will be the alternator 100% output exceeding the BIM and fuse and probably wire rating. You will probably blow a fuse.

I agree DC-DC chargers are ideal, I went with a Renogy 60 amp DC-DC bi-directional charger, this keeps chassis battery charged as well from solar or shore. 60 amps does take some time to charge a 400ah bank but keeps my 160 amp stock alternator well within operating limits with plenty left over for chassis draws as well.

You can parallel DC-DC chargers to gain more amps it just a matter of cost they are more expensive than a BIM however BIMs cannot limit current effectively.

IMO its unfortunate the previous owner removed the generator, I had the Magnum 3012 in my last 30 amp RV along with a 4000w Onan gas that could run everything including air conditioner, The inverter really not a solution offgrid compared to generator running off main fuel tank for air con. 780ah would probably run a single AC for 5 or 6 hours at most maybe all day if rig is parked in sun with the solar but you end up with low battery for night. The 3000w Magnum is not as good at heavy loads as a 4000w gen and you really don't want to idle the main engine to do the same thing with alternator, if so though you would probably want a second dedicated alternator on engine that is properly current regulated and rated.
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Old 03-30-2023, 07:29 AM   #10
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Originally Posted by Rockwood27 View Post
First, it sounds as if a LiBIM won't fully charge the battery due to its on-off function. The OP stated that he has 1440 watts of solar. If the solar is configured correctly, wouldn't it aid in topping of the batteries?
LIBIM will fully charge the battery if the alternator voltage is high enough (above 13.8v) its just a matter of time, the duty cycle of the BIM more than doubles the time needed which is the point to work the alternator less.

1440 watts will help significantly if its sunny, which isn't always the case. I know for me I avoid parking in the sun if at all possible when it warm out as the heat load goes way up. If its hot out and Im boon docking I wan to be a shaded spot but may still need to run AC, ideally somewhere cool enough to avoid AC in the shade.
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Old 03-30-2023, 09:20 AM   #11
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Originally Posted by Rockwood27 View Post
The OP stated that he has 1440 watts of solar. If the solar is configured correctly, wouldn't it aid in topping of the batteries?
Yes, for sure. I have only 300w of solar and it supplies an amazing amount of charging power for such a little bank of panels. I'd add more but seldom need much more going into my batteries.

Here's the thing, unless you are running A/C with your house batteries, and I'm not, you find your LFP (LFP = LiFePO4) batteries are so capable and recharge so quickly that more charge current is not really needed.

You don't really know and understand this coming from Lead Acid batteries. With those you are limited with depth of discharge and you're limited by slow recharge so charging is a critical issue that you need pay attention to all the time.

But with LFP batteries you are "kind of" unlimited. While depth of discharge is still a concern it's much less of a concern. Recharging goes from many hours of driving or generator to a day of sun and perhaps an hour of generator if you've really used a lot of power.

The big draw on my batteries is the residential fridge, the microwave and the furnace. But the reality is that in a day they don't use all that much of the 400aH my 2-200aH Renogy batteries have stored up. And, if the Sun is shinning my measly 300w just about totally covers it.

This is true day after day - so when I'm driving between sites my DC2DC charger is almost unnecessary. I wouldn't want to be without it - but I don't generally need it all that much.
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