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Lithium batteries 12volt 7.5 amp hour


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Hi can anyone advice me on lithium batteries. I have 2 lithium batteries 12v 7.5 amp hour for my 24 volt garage motor I need to connect in series to get 24 volt , but I have read that it is not advisable to connect lithium batteries in series , I need to use a bms to connect please advice how to connect and where can I find a bms to suit my project 

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8 hours ago, Kiranbhowan said:

Hi can anyone advice me on lithium batteries. I have 2 lithium batteries 12v 7.5 amp hour for my 24 volt garage motor I need to connect in series to get 24 volt , but I have read that it is not advisable to connect lithium batteries in series , I need to use a bms to connect please advice how to connect and where can I find a bms to suit my project 

First poviso: Safest is to check with the manufacturer/supplier. If you are in sunny RSA I assume you are referring to the "lead acid drop in replacement" type LiFePO4  batteries - locally bluenova or freedomwon (based on 7.5 as opposed to 8Ah most likely freedomwon). The batteries can be connected in series or parallel. The batteries should already each have an internal BMS to protect against over/under voltage, short circuit etc. so you do not actually need a BMS - you could however need a battery balancer to help prevent the individual batteries to become too "out of sync" in terms of charge levels. Without the balancer you likely will not blow anything up, but the total life time of the batteries could be lower

This is the bluenova info I have in terms connnecting series:

For Series configuration preparation:

Charge each battery individually till they are all fully charged.
Connect all batteries that will be in the same series string in parallel, and charge together till they are full.( draw less than 1Amp from the charger if this can be measured).
Connect in series and charge with your series configured charger.
Please take note of the batteries charge voltage and current specifications on our website, as this would be crucial for your warranty
 
Something the bluenova info does not mention is that each battery normally has a sticker on with a "batch number". Ideally you should only connect batteries from the same batch (117-2 and 117-5 is okay, 117-2 and 118-2 not really okay).
 
 
This is the info I have for freedomwon (but might not be current so check with them) (the emphasis/bold in the following procedure is freedomwon's own):
 
Should a 24V or 48V system be required it is possible to connect the 12V units in series (up to four), however this application is not covered by the Freedom Won warranty owing to too much reliance on third parties for proper installation and operation.

Should the user at his discretion decide to connect these batteries in series the following is essential for proper operation:

1. Each battery must be charged separately up to exactly 14.2V on the same day prior to being connected in series to ensure that they are equally charged (balanced) prior to use.

2. The cables connecting each battery to the next must not be shorter than 200mm to reduce heat transfer from one terminal to another. The interconnecting cables must all be the same size and length and the size must comply with Table 4.1 above.

3. These batteries must be fitted with a battery balancer (or three as in below 4 series example) when connected in series.

4. When operated in series the maximum current values provided in the specification sheet must be de-rated to 70% to cater for series connection dynamics between the batteries e.g. Different battery internal resistance values.

5. The max charge voltage setpoints must be set correctly to ensure that none of the batteries reach its maximum cut out voltage: 5.1 Two in series – 28V maximum charge voltage

5.2 Three in series – 42V maximum charge voltage

5.3 Four in series – 56V maximum charge voltage

 

some other things to consider:

if possible check the actual startup and running current of the motor. Unlike some of the bigger 12V lithium batteries and the pylontech etc. type batteries the bluenova 7Ah type battery is only rated 1.44A continuous and 7.2A burst for 30 seconds (not the expected 1C continuous and 2C surge). The freedomwon spec sheet still shows 1C continuous and 2C pulse <3sec - again confirm with them (the bluenova info is dated August 2020 but freedomwon info is older and specs could have changed). For some refernce the hungrier of my two Centurion X-Tracs draws about 2.66A on the initial "pull" - this is for a single alluminium sectional door - type of door (tilt, sectional, roller), weight, condition of springs, balance, lubrication, motor spec etc. will all influence that number). The lamp accounts for about 0.5A. When just sittng there looks like the unit draws about 0.1A. 

You may need to replace the connectors on the existing garage motor battery wiring. The bluenovas I have, have 6.3mm connection tabs, while standard lead acid 7Ah batteries tend to have 4.8mm tabs.

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I concur with the previous post and would like to add one more thing. For the Bluenova I had a chat with a support guy and he says that the absolute minimum voltage for the internal BMS to start balancing cells is 13.7V. I replaced an alarm system battery with one of these and had to modify the charge circuit a bit to get it over 13.7V, I think it was initially 13.6V for the lead-acid batteries. While doing that I increased it to 14V just to be safe and that the internal BMS can happily balance the cells when needed.

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15 minutes ago, Ingo said:

... I think it was initially 13.6V for the lead-acid batteries. While doing that I increased it to 14V just to be safe and that the internal BMS can happily balance the cells when needed.

temporary hijack.. thanks for the balancing voltage info. Is your alarm in essence now floating at 14V? I decided to be happy with a 13.6'ishV since the alarm battery is actually used very little (truly a "standby battery") so spends majority of the time floating. Wonder which will have the most effect on shortening lifespan - not balancing often or sustained higher voltage?

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Well, the datasheet doesn't specify a specific voltage for standby, it just gives a range. Yes it will work on 13.8V but the recommendation from Bluenova support suggested 14V. I also wanted to use 13.8V initially, and will probably do that in future because like you said, this is a standby battery after all. The downside is that I will never get a Low Batt warning on my system, it's calibrated for lead-acid but I can live with that.

Edit: Here is a quote from Bluenova "The rated charge voltage for the mobile power series batteries are 14.2 V for the bulk charging and 13.7V for the float voltage."

This is a test system so I can see what life I can get out of this. Normal lead-acids don't last all that long, maybe three to four years. That said, my garage motor 24V lead-acid pair (with balancer) is on 5 years now and still going strong - best purchase ever, the balancer for that battery pack I mean.

Edited by Ingo
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19 minutes ago, Ingo said:

Well, the datasheet doesn't specify a specific voltage for standby, it just gives a range. Yes it will work on 13.8V but the recommendation from Bluenova support suggested 14V. I also wanted to use 13.8V initially, and will probably do that in future because like you said, this is a standby battery after all. The downside is that I will never get a Low Batt warning on my system, it's calibrated for lead-acid but I can live with that.

Edit: Here is a quote from Bluenova "The rated charge voltage for the mobile power series batteries are 14.2 V for the bulk charging and 13.7V for the float voltage."

My batteries are also bit of test case. Off-topic, but bluenova info does vary a lot - maybe because the internet has a long memory. Bluenova actually have a "customer self service portal", where you can access battery manuals etc. and the "MPS series" guide from there shows 13.9V float

MPS_Series_Charge_Discharge_Settings.jpg.a35ca467fa93b68ddaf28656091e947a.jpg

any case, I will see in time what happens (and if I start messing with the charging circuits I know the magic smoke will be released from somewhere...)

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