jray3
Posts: 1648
Joined: Tue Dec 06, 2011 1:05 am
Location: Tacoma area, WA
Contact: Website

Using a second pack in parallel

Thu Sep 12, 2019 2:53 pm

I'm working a new job 75 miles away, and can get there with one DCFC stop, but would of course prefer not to, and I happen to have my original battery pack just hanging around, with probably 10 good kWh available in it...
I recall but am not finding the thread on a European college project that placed a second battery pack inside the car and ran both together. My desire is much simpler, to place the old pack on a trailer and use it for part-time range extension, without messing up the BMS, capacity calculations, etc...,

SO, discussion please on how to accomplish this. Most lokelyly, I could charge the battery trailer with an external charger and then connect in parallel where the DC feeds into the inverter (via a pigtail with an Anderson connector). However, the ideal solution would be to recharge in parallel with the OBC or/and DCFC.

Just like getting extra regen (and range) with a pusher trailer, once the batteries are hooked up in parallel, the car may not be any the wiser if the external battery is hooked in downstream of the point at which coulombs are counted/ amp-hrs are measured. BUT, unlike a pusher trailer, I won't simply be unloading the motor so the car has to do less work and gets more regen, as if I'm always driving downhill. In the case of a battery trailer, the motor will be working harder, pulling more amps and amp-hrs, while the battery mysteriously delivers more energy than expected, and the cell voltages stay healthier for longer.... Since the BMS is very slow to change AH measurements and turtle mode is based on the lowest V cell measurement in the onboard pack, I expect that part-time use of a range trailer wouldn't cause overcharging or overdischarge problems for the onboard pack (especially if one employs common sense and doesn't try to drive 80 miles with the optomistic RR numbers shown after disconnecting the trailer!).

Any good links or ideas on where to connect in the offboard pack, and how to energize the offboard BMS?

-Jay
2012 i-SE "MR BEAN" 105,400 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
1983 Mazda RX-7 EV 43 kW 10 kWh
1971 "Karmann Eclectric" EV 240 kW 19 kWh
1965 Karmann Ghia Cabriolet

jray3
Posts: 1648
Joined: Tue Dec 06, 2011 1:05 am
Location: Tacoma area, WA
Contact: Website

Re: Using a second pack in parallel

Sat Sep 14, 2019 9:01 pm

viewtopic.php?f=23&t=3074&hilit=Wrecked+pack+discussion&start=20
Here’s that thread, wow 4 yrs ago, sure would like to hear the rest of the story...
2012 i-SE "MR BEAN" 105,400 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
1983 Mazda RX-7 EV 43 kW 10 kWh
1971 "Karmann Eclectric" EV 240 kW 19 kWh
1965 Karmann Ghia Cabriolet

Gary12345
Posts: 42
Joined: Tue Feb 12, 2019 4:15 am
Location: Essex, UK

Re: Using a second pack in parallel

Mon Sep 16, 2019 7:21 am

You need to be incredibly careful.... we are talking 50Kw at 400v...

You'd need to be sure that your connectors were extremely secure and cope with the power without getting hot, water ingress is also a factor due to rain.

There is also the issue that when you are not using your range extending trailer you have a powered up connector which is not connected to anything - you can't just leave that dangling down towards the road ! Also need some way when you are using the range extending trailer to "turn on" the contactors in the 2nd pack, you cant have them on all the time. You will also be using quite long cables so cable sizing and robustness comes into it, since the cable will be bending as the trailer pivots.

Charging the 2nd pack will be interesting since you don't have the car's BMS to control the balancing - if I remember correctly with that original thread they could swap which pack the car was talking to (i.e. connect the comms to the other pack) and therefore the BMS's in the 2nd pack worked for charging (one pack at a time).

I believe they said in that other thread that handling was affected, I doubt it was ever used long term, more of a technical experiment. Don't get me wrong this is possible but you have to make sure what you are doing is safe.

I've researched a few EV range extenders, they often us a dc-dc boost converter with say 5 or 10kwh of batteries, boost the voltage out from those batteries up to the correct voltage for the car and connect it to the motor controller input, something like a 2kwh dc-dc boost converter gives a charge effect a bit like regen all the time.

The point of connection and the effect you describe, where the car just thinks it is using less energy to move are all correct, you'd feed power into the motor controller battery input.

You can't charge 2 packs at the same time as the BMS in the car tells the CMUs in the battery pack which cell to bleed during balancing, so its a 2 way communication. It can't talk to 2 at once, you'd be better charging the 2nd back separately. As far as fast charging that is just not possible i'm afraid, aside from the electrical issues you can't cool the 2nd pack as the car does under fast charging.

Thanks.

JoeS
Site Moderator
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Joined: Thu Dec 15, 2011 5:47 am
Location: Silicon Valley, California

Re: Using a second pack in parallel

Mon Sep 16, 2019 8:27 am

Gary12345, points acknowledged. You might note that jray3 has performed many EV conversions, owns a number of such vehicles, and has worked with high school students for years on such projects, so is tuned in to the safety aspects of such endeavours.

In the US, the most popular connectors for such high power and high voltage projects are made by Anderson - for example, their high current SB series has been used for years and is available with a number of accessories, such as protective caps.

jray3's concept is to not put the second battery inside the car but to have it on a small trailer which he'd be towing. He has the complete and fully operational battery pack that he recently swapped out of his i-MiEV - it just has a reduced capacity. The concept is simple, but the devil is in the details primarily due to the need to integrate with the existing car's design.

jray3, I know you're under the gun to get this implemented soon as a 75-mile one-way commute in the i-MiEV, but having to rely on DCFC isn't viable for long. Sorry, I can't offer any meaningful suggestions. Good luck!
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV

Gary12345
Posts: 42
Joined: Tue Feb 12, 2019 4:15 am
Location: Essex, UK

Re: Using a second pack in parallel

Tue Sep 17, 2019 5:44 am

Hi there,

Good to hear you have some previous experience, I worry a bit when I read a question like that !

Even using an Anderson SB with a cover you can't really consider that safe to be powered up when not in use - the only safe way I can see this working is to install a contactor box in the car and add leads from the battery controller input to the contactors. Then coming out from this you could have the anderson connector. You'd need to do the same at the battery end, here you could reuse the contactors in the pack. This means that when you are connecting the 2nd pack you are not handling live connectors.

This is the only way I can see it being safe, you'd then build a simple control circuit to energise the contactors. In terms of startup procedure, you need to bring the car to READY then flip the switch to engage the contactors before you move off, that lets the car do its safety tests and if its happy to fire up then it closes the contactors in the main pack and then you manually engage the ones for the 2nd pack. It may be prudent to make the control circuit only able to be turned on when the car is on, then it could automatically disconnect when the car is turned off.

You'd need to think about making sure the 2nd pack in the trailer doesn't get too hot - you can't leave it exposed to the sun. May be an idea to get the fan in the back of the 2nd pack running while you are driving if the climate there is hot and monitor temperatures inside.

It would be a lot of work but certainly achievable if you have the appetite.

Good luck !

1kaspars
Posts: 17
Joined: Tue Mar 28, 2017 10:39 pm
Location: Latvia

Re: Using a second pack in parallel

Tue Sep 17, 2019 7:11 am

How about adding ~3kWh "powerbank" made from remaining LEV50? Should be enough for 75miles combined.
32s cells + (BMS?) + DC/DC 115V to 360V @10A paralel to on-board charger?
55+ kg

+ maybe 12V charging not from main battery.
EU 2012 I-MIEV owner for 24months, 24'000 miles driven, odom 56'500miles, 38.5Ah left

jray3
Posts: 1648
Joined: Tue Dec 06, 2011 1:05 am
Location: Tacoma area, WA
Contact: Website

Re: Using a second pack in parallel

Wed Sep 18, 2019 10:53 pm

Thank you Gary12345 and 1kaspars
I wouldn’t leave 360V exposed in a dangling Anderson connector, but energize the trailer pack’s CHAdeMO contactors with 12V switched from the driver’s seat, so as to parallel the packs only in READY mode.
Recharging with an offboard charger is easy enough, but I’d want to figure a way to connect the BMU at least every few cycles for some balancing.
This is fairly easy with LEAF packs due to the handy LEAF Pack sniffer by Wolftronix.
http://www.wolftronix.com/packSniffer/index.html
Not only does it output full pack data from a “disembodied” battery, but to do so, it wakes up the BMS inside the LEAF pack. As long as 5V is input to the sniffer, the LEAF battery will balance, and his second version will even control the contactors, but no way I’m hooking up a 380+ Volt LEAF pack to my I-MiEV!
Here’s me using the prototype sniffer a few years ago.
https://m.youtube.com/watch?v=gGcYBvNPxB4
And the full post...
http://karmanneclectric.blogspot.com/20 ... f.html?m=1
2012 i-SE "MR BEAN" 105,400 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
1983 Mazda RX-7 EV 43 kW 10 kWh
1971 "Karmann Eclectric" EV 240 kW 19 kWh
1965 Karmann Ghia Cabriolet

jray3
Posts: 1648
Joined: Tue Dec 06, 2011 1:05 am
Location: Tacoma area, WA
Contact: Website

Re: Using a second pack in parallel

Wed Sep 18, 2019 11:04 pm

1kaspars, here’s an example using a low V battery and a UPS inverter to charge an I-MiEV (from its own back seat).
https://m.youtube.com/watch?v=NlZ6BOzn41g
I do similar off-grid charging from the big 48V battery in my tiny home with a 3kW pure sine wave inverter, but of course towing the 3800lb tiny home is not an effective means of range extension!!
https://m.facebook.com/groups/634985806 ... 4468027106
Last edited by jray3 on Thu Sep 19, 2019 12:09 am, edited 1 time in total.
2012 i-SE "MR BEAN" 105,400 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
1983 Mazda RX-7 EV 43 kW 10 kWh
1971 "Karmann Eclectric" EV 240 kW 19 kWh
1965 Karmann Ghia Cabriolet

jray3
Posts: 1648
Joined: Tue Dec 06, 2011 1:05 am
Location: Tacoma area, WA
Contact: Website

Re: Using a second pack in parallel

Wed Sep 18, 2019 11:41 pm

Here are examples of the sort of battery trailer I am proposing.
First we have two RAV4 packs in parallel, so much more complex due to liquid cooling, etc. well done with lots of videos, just a bit verbose, so the first link is a succinct summary from Rich Rebuilds.
https://m.youtube.com/watch?v=Qb7vg08zl ... e=youtu.be

The maker himself posted 17 videos on the first trailer alone! The second version includes a foamier camper body.
https://m.youtube.com/watch?v=sZ4wy_hIN ... 4&index=16
2012 i-SE "MR BEAN" 105,400 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
1983 Mazda RX-7 EV 43 kW 10 kWh
1971 "Karmann Eclectric" EV 240 kW 19 kWh
1965 Karmann Ghia Cabriolet

Gary12345
Posts: 42
Joined: Tue Feb 12, 2019 4:15 am
Location: Essex, UK

Re: Using a second pack in parallel

Thu Sep 19, 2019 8:04 am

Its a lot of complex work to have a trailer like that RAV4 one.

What I have considered a couple of times is how much power is consumed on the 12v side during a full discharge. What would be fairly easy would be separating the 12v system so that the dc-dc converter does not top up the 12v battery during driving. Then you replace the 12v battery with 2 or 3kwh of lithium batteries and only charge them when the vehicle is on charge.

This reduces load on the main traction battery while driving as the 12v systems are drawing nothing from there, they are draining their own mini pack. Of course you'd need to make sure the 12v system did not go flat, and there would be a lot more load from night/rain driving from lights and wipers but if someone could measure the 12v load from the dc-dc during normal driving that would be very interesting.

For keeping the main battery topped up it would be a lot easier to get the LEV50 cells out of your old pack, putting them in a 7 cell or 14 cell series configuration for 24 or 48v and get hold of a dc boost transformer to get it up to the main pack voltage. Of course you can put multiple series strings of cells in parallel for more capacity. That is how commercial range extenders tend to work. It is probably easier (but less efficient) to use an inverter to get to something like 240v then a dc rectifier to get to the right voltage, from the specs of commercial setups they go with 2kw boost circuits. Installing that in the boot would be viable.

Thanks.

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