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--- electrical:12v:self-jumpstarting [2024/02/19 02:39]
frater_secessus [maintaining starter batteries] gotchas
+++ electrical:12v:self-jumpstarting [2024/03/22 08:19] (current)
frater_secessus [Self-jumpstarting]
@@ Line 3: / Line 3: @@
 Self-jumpstarting means jumpstarting one's own vehicle without the use of another vehicle.  It's done by [[electrical:12v:alternator|combining]] house and chassis batteries by one of several methods discussed below.
-Note:  regardless of specific method used, it will be gentlest on the system to **combine the batteries and wait a few minutes for the starter battery to come up** instead of immediately attempting to start. This will minimize the amount of current flowing from house bank to vehicle chassis. This may be especially important with Li chemistries that have BMS protections.
+Note:  regardless of specific method used, it will be gentlest on the system to **combine the batteries and wait a few minutes for the starter battery to come up** instead of immediately attempting to start. This will minimize((or eliminate, if the connection is broken prior to starting)) the amount of current flowing from house bank to vehicle chassis. This may be especially important with Li chemistries that have BMS protections.
 ===== jumping from the house bank =====
@@ Line 43: / Line 43: @@
 Note the Smartpass does this when added to the CTEK D250-series chargers.
 ===== maintaining starter batteries =====
@@ Line 49: / Line 51: @@
 Some standalone solar charge controllers like the [[https://amzn.to/2PPTD2L|Morningstar SunSaver Duo]] can, for example,  charge the start battery with 10% of solar harvest and the house battery with 90% using different charging profiles.  As with DC-DC above, it's not self-jumpstarting but can help prevent the dead starter battery.
-[[electrical:12v:alternator#voltage-sensing_relays|dVSR isolators]] (including [[electrical:12v:alternator#lithium-specific_vsr|the Li-BIM]]) automatically connect the banks when the voltage on either side reaches the defined setpoing.
+[[electrical:12v:alternator#voltage-sensing_relays|dVSR isolators]] (including [[electrical:12v:alternator#lithium-specific_vsr|the Li-BIM]]) automatically connect the banks when the voltage on either side reaches defined setpoints.
 Some Victron Multiplus models have a 4A starter battery maintenance feature.
@@ Line 64: / Line 66: @@
 Also see [[electrical:12v:alternator#gotchas|the subarticle on combiner gotchas]].
+==== starter battery chargers alongside DC-DC chargers ====
+If a DC-DC is present and relatively weak shore power charging is applied to the starter battery there are circumstances where the DC-DC can cycle on-off constantly.  Let's consider a 20A DC-DC and a 10A starter battery maintainer.
-===== jump packs =====
+  - starter battery at 12.7v before charging applied
-[[https://amzn.to/2ITeBba|Lithium "jump packs"]] have become more popular.  Lithium-chemistry batteries can discharge quickly, so even relative small packs can jumpstart a car.
+  - starter battery voltage rises from 10A charging
+  - DC-DC sees >13.4v and [[electrical:12v:b2b#sizing|starts pulling ~22A]] to charge house battery at 20A
+  - starter battery drops to 12.6v and DC-DC charger stops
+   - repeat forever
-'Dwellers may want to pick a jump pack that has 12v or USB outlets so it can have other uses.
+To avoid this scenario either:
+  * ensure the DC-DC rating is smaller than the starter battery rating; or
+  * apply the shore power charging to the house bank side instead((for DC-DC models with starter battery maintenance, see above))
+===== waking lithium batteries =====
+Lithium banks can go dormant at low voltages in order to protect themselves:
+  - the BMS turns off the DISCHARGE channel to keep cell voltage from going any lower
+  - so the battery internally has ~11.0v or whatever but does not pass that voltage to the terminals on the outside of the battery case
+  - which means smart chargers don't see battery voltage
+  - and think there's a problem so they won't start(("too smart for their own good"))
+What we need is a //dumb// voltage source to get the party restarted.  The starter battery will do nicely.
+  * Rigs with IGN-triggered relays can briefly turn the key to ACC then back off.((don't leave it on ACC long or the dead Li will suck power from the starter battery.  Either turn ACC off again or actually start the vehicle.))
+  * Rigs with voltage-sensing relays will have to actually start the engine or press a manual override switch to activate the VSR and wake the lithium bank.
+  * Rigs with diode- or FET-based isolators would start the engine to spin the alternator and get power flowing through the isolator to the sleeping lithium
+===== jump packs =====
+[[https://amzn.to/2ITeBba|Lithium "jump packs"]] have become more popular.  Lithium-chemistry batteries can discharge quickly, so even relative small packs can jumpstart a car.
+'Dwellers may want to pick a jump pack that has 12v or USB outlets so it can have other uses.

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