Switchable voltage level?
 

I am figuring out what battery voltage would be best performance and pocketbook wise, and they are a little different. For 100 aH LiFePO4 batteries, 120V would cost ~$4,600 (before shipping and cables), and 156V would cost ~$5900. Now, I can either save for another two months and buy the 11 extra battery packs (which will probably happen next spring).

But I was thinking, what if I bought 3 12V deep cycle batteries and could use a relay to switch them in when I needed some more power? Yet could disconnect them when cruising down the highway.

it could work, but complicates operation and charging substantially, and isn't free as you have to get lead/charger/operational controls and time to sort it out. I might just build it at 120v and save room for the extra batteries.

Now that I think about it, it does sound complicated to charge and setup to save maybe $800 for a few months.

My next thought might be interesting. Would it be possible to have 156V worth of LiFePO batteries, but only run 120V normally. Then if you need it, switch to 156V. But, could you also swap 36V (drained) out of the 120V for the charged 36V to get better range if you need it?

I think that only complicates things. Your controller can probably manage 156v more efficiently than 120 given the fixed voltage drops will be less of the whole, etc.

A relay that can handle that voltage and the amps that you are going to draw is not going to be cheap either, I'm also not sure how the internal resistance of the lead acid would compare to the lithium, you would also destroy the battery pack that discharges first if you draw it down to far, much more so then if you just run your pack dead in a normal fashion.

Hi Caps18, You have an interesting idea here,
From what i understand you thinking about having lithium and lead acid batteries in series running off the same charger at 156v. With an optional switch to just charge the lithium batteries at a 120v.
I am under the impression that lithium batteries (at least consumer batteries) require special charging methods compared to lead acid batteries
if your interested in knowing anything and everything about batteries this website has heaps of info about all battery types.
Charging lithium-ion batteries

The maximum current draw and supply of the battery bank will become the minimum of the lead acid or the lithium. So if the lead acid batteries can not supply much current the lead acid batteries will limit the current supply to the motor and the current charging rate. I havn't checked the max currents of lithium and lead acid batteries so not sure if its a problem but this might be something you want to look at.

Also running a charger and motor at 2 different voltages seems like a considerable amount of work

Regarding your 2nd comment about running only lithium batteries but at 2 different voltages. I like the idea of switching out discharged batteries. Some time ago i did a study for work on the concept that the better place was looking at. details can be found here for anyone interested
Better Place | The Global Provider of EV Networks and Services.
They are looking at having service station like locations which swap your whole battery bank for when you need to drive further than just say 100miles which the standard battery bank is capable of. Very cool idea and I think this idea has potential to make electric cars become more mainstream
So its all about decreasing the weight of the car which is why i like your idea of adding or removing batteries from your bank based on what distance you want to go. Unfortunately it is very difficult to do. If you swap in and out batteries in series the motor controller needs to be able to deal with the higher or lower input voltage which may compromise efficiency or abilities of the controller.
A better method would be to swap in another 120v rack of batteries which you connect in parallel. There may be issues in charging 2 paralleled battery packs, i think they will share current but i cant be sure without looking into it. Having 2 battery packs means you could connect up 1, when it discharges a switch / relay disconnects that battery pack and connects up the 2nd battery pack. So you can remove only the depleated pack.
Doing the same thing when the batteries are in series is increadibly difficult if not impossible. You can not make a set of given batteries discharge without the others discharging at the same level (there might be a way if you have electronic connection to each and every battery but thinking on the spot now i cant think of an way to do this)
Also regarding your comment at running at 156v pack at 120v is a good idea but in practice will significantly reduce the Ah of your pack when running at 120v. Batterys have a narrow band of voltage at which they can supply current, for example look at your standard consumer batteries you will get charge from them between say 2.2v and 1.8v. When you get below 1.8 v they have already lost almost all of their energy. So a 156v battery pack at 156v has 100% of its charge. The same battery pack at 120v may have only 20% of its charge.
Note also that some types of batteries will become perminately damaged if their charge drops below 20% which is why if you ever fully discharge your car battery a few times its never the same and you usually have to get a new one. Sorry i cant remember if lithium has the same characteristic.

Hope i havn't discouraged you, you have some great ideas here so i hope you find a way to achieve what you want. Look forward to seeing what you end up doing

I like this idea best
I do this on a electric bike, 12v propulsion booster switch wired in series with the output from the controller. (SPDT On-On)