# Lithium Ion and Lead Acid battery in series

I have a 12v trolling motor (55 lb) that is rated for 50 amps. At speed I only pull < 30 amps in my inflatable boat. I know that increasing the voltage will make the prop spin faster, which will draw more amps, but also make me go faster, which is the goal.

Is it possible/safe/feasible to connect my 12v lead-acid battery in series with a 3.7v Lithium-Ion bundle (of reasonably similar C) for a 15.7 (nominal) volt setup? I have already done some hand-wavy calculations and think I will hit my amp limit (though I should probably stay around 45 to be safe) at ~14.5v, so I will use a PWM (which I already have installed) to limit my amperage... I.E., dont worry about the other factors involved;

I just want to know, is there some fundamental reason such a setup would not work?

I have already considered using step-up/boost converters to increase the voltage, but with the amperage required I would need either an expensive one or many less expensive ones, and I rather spend that money/use that space on more battery power because of the added Watt-hrs they bring onboard.

Cheers, Bryan

• The current is determined by the load. Put some weight into your boat, pull again and measure the current. – Janka Nov 1 '16 at 23:36
• If your motor is only pulling 30A at 12V , you just need a bigger prop pitch. – Sunnyskyguy EE75 Nov 1 '16 at 23:36
• They are hard to find for trolling motors. My current prop is 3.5" pitch, and I only know because I measured. That kind of information is almost impossible to find. Spinning it faster should work. – Bryan Goggin Nov 1 '16 at 23:42
• Trolling is not approved of on this list :-). – Russell McMahon Nov 2 '16 at 5:56
• In my opinion it is not feasible for the simple reason that charging will be complicated and difficult. You will probably need to disconnect the batteries and charge the lead with a lead acid charger while charging the lithium with a lithium charger. During discharge you will really have to watch both the lead voltage and the lithium voltage separately to make sure you stop before you get too low. If you ever forget to do that, you will very likely damage one of the batteries. If you do it several times, there may be permanent noticeable decrease in capacity. – mkeith Nov 2 '16 at 7:08

## 1 Answer

What you propose should work technically and is probably not too hard to do "well enough" as long as you are sensible [tm].

As Peter says, mixing battery chemistries can often have bad outcomes, but as long as you do not violate basic requirements this could be reasonably problem free.

It is not obvious why you would use a LiIon second battery - lead acid will better match the cost/capacity and general cycle lifetime for given use.
If your main battery was say about 50 Ah (less than 1 hour at 50 A due to C rating usually being at 10 hour rate or even lower) and you wanted both batteries to work together throughout the LiIon battery would also need to be 50 Ah or more. That's not a small battery and you are going to need a separate specialist charger to charge it - which is OK if you have the equipment already - but otherwise adds expense and complexity. You may be able to obtain 2V indivual LA cells or a 4V battery (they do exist) but again are faced with the charging issues mentioned below.

A LiIon battery MUST have it's low voltage discharge limited to a safe voltage. It MUST be rated to discharge safely and continuously at say 50A.
Max charging current is liable to be 25A or 50A (or other - varies with manufacturer). You COULD safely enough [tm] charge a single LiIon cell by limiting Imax to at or below rated max value and setting maximum charge voltage to say 4V or slightly less. You can then allow it to charge until 4V is reached and can float it at 4V "safely". This charges to noticeably below max capacity and increases cycle life. You MUST NOT charge it all the way to 4.2V and float it there - battery death happens soonish.

Note that LiIon will have 4.2V (if fully charged) to start and 3V or so fully discharged. (Lower possible but unwise if long cell life is wanted).

• I don't know. My feeling is that sooner or later a person will not do everything right and ruin one of the batteries. Either the Lead or the Lithium. – mkeith Nov 2 '16 at 7:04
• @mkeith I agree :-( :-). As in " ... but as long as you do not violate basic requirements ...". Murphy takes this sort of arrangement as a challenge, ala. – Russell McMahon Nov 2 '16 at 9:00
• Thank you for this response. Am I correct in my understanding that I can add C values for LiIon batteries in parallel? If I go with a Sanyo, etc 18650 with say 2500 mAh capacity, I need 20 to match the 50 Ah of my lead acid? The main reason for LiIon is weight/size and the voltage is reasonable. 2v LA cells are usually very expensive and a bit overkill. – Bryan Goggin Nov 2 '16 at 13:32
• And then if I have 20, can I divide current draw of 50 amps across the 20 so each cell will only be flowing ~2.5 A? I am not sure how that works. – Bryan Goggin Nov 2 '16 at 13:38
• @BryanGoggin No no no! :-). Notionally you can add Ah in parallel BUT combining even a few cells in parallel has issues in practice and more than a few can lead to very bad things in practice. People sell single large capacity LiIon and LiFePO$cells - 20 40 80 100 ... Ah !. One of those would work OK. BUT note mkeith and my comments on needing to charge the batteries separately and monitor voltage etc. 5 x LiIon or LiFePO4 cells would be better. Also hi$ entry level but lower whole of life cost if treated well. I lean towards LiFePO4 as longer life, more benign under stress. Generally nicer. – Russell McMahon Nov 2 '16 at 22:03