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Mixed Lithium and LA


dmr

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I know the Lithium advocates love to talk about lithium batteries so I offer an opportunity 😀.

 

We have six Trojan T105's in a sort of semi-split bank (4+2).

The 4 are just about finished (just under 7 years on an off grid livaboard so I don't think a warranty claim will work 😀)

My plan is to buy four new T125's but I have this little nagging feeling that I should go lithium and mix them with the two good Trojans.

I can sort of see how this works but have two problems.....

The Trojans really like 14.8 or even 15volts to charge and this is not really compatible with the Lithiums???

I can see that the lithiums discharge first, which is good, but they also charge first, so if we have got the whole lot a bit low and discharged the LA's its going to be an epic to get the LA's charges because the lithium would take priority. Is there an clever/easy way to give charging priority to the LA's???.

I can think of custom electronics, big solenoids and dc-dc converters etc etc but really have no time for any big projects for the next year or two.

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Well you could have an isolation switch and manually disconnect the Li when you need to fully charge the LA. But why bother, why not just go for Li on its own? You have already overcome one problem IIRC, you have an alternator controller with temperature protection and adjustable voltage?
 

I’ve always thought mixing Li with LA was for those faint of heart who lacked confidence and struggled to make decisions. A bit like people who drink rosé wine.

Edited by nicknorman
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As Nick says, I just have a switch in mine. 

 

With 560W of solar I find LAs are fine most of the year. Still on those knackered old Yuasas the Smartgauge messed up. They only need to last overnight then the solar comes back on and takes over. So most of the time I have the lithiums switched off and I only need to turn them ON if/when the LAs get too low and need charging after 8pm. The lithiums work fine to tide them over the night. 

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20 minutes ago, nicknorman said:

Well you could have an isolation switch and manually disconnect the Li when you need to fully charge the LA. But why bother, why not just go for Li on its own? You have already overcome one problem IIRC, you have an alternator controller with temperature protection and adjustable voltage?
 

I’ve always thought mixing Li with LA was for those faint of heart who lacked confidence and struggled to make decisions. A bit like people who drink rosé wine.

 

But wouldn't I need a more advanced alternator controller to prevent over-charging of the Lithiums...which would involve communications with any BMS inside the lithiums.?

I thought the big advantage of the LA's was to prevent any huge voltage spike if the Lithiums disconnected when fully charged???

 

and what would I do with the 10 litres of deionised water that I've still got ? 😀

 

How does the Ssmartgage cope with a hybrid lithium-LA bank??? does it show 100% till the LA's start to discharge then give a correct reading? or does it just get really upset?

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21 minutes ago, dmr said:

 

But wouldn't I need a more advanced alternator controller to prevent over-charging of the Lithiums...which would involve communications with any BMS inside the lithiums.?

I thought the big advantage of the LA's was to prevent any huge voltage spike if the Lithiums disconnected when fully charged???

 

and what would I do with the 10 litres of deionised water that I've still got ? 😀

 

How does the Ssmartgage cope with a hybrid lithium-LA bank??? does it show 100% till the LA's start to discharge then give a correct reading? or does it just get really upset?

Ideally yes, but if you set the alternator voltage to about 14.3 it won’t over charge in the short term. What would be good is an ability to stop charging, ie an alternator field current disconnect switch - you’d have to do it manually but better than nothing.

 

Sterling do an alternator load dump absorber (although I made my own with transient absorption diodes) and anyway the disconnect only happens if something has gone badly wrong.

 

If you go for lithiums, you need to move to Ah counting. Smartgauge doesn’t play.

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When charging if you disconnect the lithiums when the charge voltage reaches a set point around 14 volts, depends on how hard you want to charge the lithiums, the still connected lead acid will then continue to charge as they do now until the volts reach 14.X and the tail current is below 1%. When you stop the charging, as the surface charge on the Lead is dissipated and their voltage falls below 13.6 volts you can reconnect the lithium to take over the running of things off the batteries.  Provided you have a BMS that can be programmed to turn off at 14V and turn on at 13.6 it is all done for you.

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